minimum budget for 30 days trip plan?

Actually I want to go to a 30 days trip to Japan. And I am not getting sure that how much budget should I have.

That not not a simple answer.
Depends on many factors.
Food you eat.
How much you travel.
Where you wanna visit?

Do some homework. Plan where you will visit in Japan. Price hotels, airfare and train travel based on that. Plan some activities and look up what they cost on the web. Add in a shopping budget. Then food will depend on where you eat but I�fd say 5000-8000 yen per day is average.

All this information is available online.

I�fd say low budget estimate is 5000 yen per person per day plus lodging , airfare , transportation.

Minimal? Depends on what you mean. I've both travelled in Japan extensively in a genuinely minimal fashion, but I can easily occupy myself with things that don't cost much money if needs be. It may not be for you.

This is based on what I would do if I were to visit one big city with a few out of town day trips (not too far e.g. Tokyo to Kamakura or Nikko, Osaka to Himeji). The only way to be truly economical with accommodation costs is to limit yourself to one city. Understand if you want to travel around it's going to cost A LOT more.

1 month in a gaijin house (monthly mansion) - 80,000 to 100,000 yen for a self-contained apartment, much less (50,000?) if you're willing to share kitchen, bathroom, etc. with others. Companies like Oakhouse, Sakura House, etc. are decent enough. It's more expensive than renting a normal apartment like you would if you lived in Japan, but still only a third (potentially less) the price of staying in a business hotel for a month)
Food - 60,000 yen (2,000 per day). Most people who visit and stay in a hotel are forced to eat out almost all the time. You're staying in a gaijin house which will have kitchen facilities. If you can cook then you can feed yourself for less than 1,000 yen per day, but you're travelling and probably want to eat out sometimes so I've doubled it.
Transport - 30,000 yen (1,000 per day). Again, you could get away with less and it totally depends on what you want to do. I could easily enjoy myself for a month in a big city with less than 1,000 yen per day for transport.
Everything else - Basically impossible to say but anything from 30,000 to 140,000 yen depending on what you want to do, spend money on, etc. In my opinion 5,000 yen per day for 30 days would amount to a really good, fulfilling trip.

Adding all this up it could be anything from 170,000 to 330,000 yen not including flights. Even at the higher end I would consider this very reasonable for a month in a country like Japan. I can (and have) done it for well within this range, and always have the time of my life, so even though some people will tell you Japan is the most expensive place on earth it's totally doable if you're anywhere near as easy to please as I am.

I am also currently planing my next trip (a month, like you) and my budget is 360,000, but that's because I've decided to splash out a little bit to reward myself for something I did last year. I have done long-term travel in Japan for less than 200,000 per month though.

Well...
As the other writers above say, it depends a lot of your preferences for your trip.
So you can spend a lot more money or you could be very fine with roundabout 1.000 $ for 7 days...

For example:
How many days do you like to visit?
How many towns do you like to visit? ( plus costs for travel between this towns. saving money with Japan Rail pass is possible?)
What do you want to see? (Museums, Gardens, Tokyo Monster Café etc., costs for travel from one to another, entree fees)
Which standard do you wish for your Hotel(s)? (Capsule Hotel, Hostel, Bussiness Hotel, "fancy instagrammable" Place, etc.)
Food? (Any specialities do you want to eat? Diet restrictions?...)
Shopping? (Any special things you want to buy? Collector Figurines, Pokemon Merch, ...)
What time of the Year do you like to travel?
And so on...

If I plan my trips, I rougly take first notes to the questions above, than I research for prices for Hotels and flights for the time (and length) I want to travel. Helpful could be sites like "Booking" or "Swoodoo".
For the places I want to visit, I take notes about Entree Fees and travel costs ("google maps" are very helpful for travel instructions and costs).
If you are looking for interesting tipps for food (and costs), there are many youtube Videos to watch or simply google for this.
You get also nice shopping ideas on youtube ("Japan Haul" Videos...)

I hope this is a little helpful for your planing!

I usually figure about $120/day, plus my JR rail pass, large attractions (Disney Sea, Fuji Q, a bus tour, or maybe a cable car or boat ride), and fancy resorts/hot springs.

If you really wanted to go minimal, say no attractions, simply walking around for transportation, say four kilos of trail mix, two kilos of jerky, a jar of peanut butter, and a loaf of bread; your main cost will your accommodations. My least cost was about $25/night, for a 3 tatami mat room in Tokyo, with a communal bath and shower on the floor above. Is that really how you want to live, after paying for airline tickets to get to Japan?

Your main costs are food, shelter, transportation, attractions, and souvenirs. Do not assume that getting a 7 or 14 day JR pass covers any and all transportation costs, either. If, for instance, you plan to visit Yakushima Island, you are most likely going to have to pay for a Hydrofoil boat ride from Kagoshima. You will also find that local subway and busses are not covered by the pass. In Kyoto, for instance, my JR pass will collect dust in the hotel room, and will get a day bus pass to get around.

This trip is best fleshed out by you, doing some research that revolves around your interests. You may love the ocean, the city, or the mountains. Suggesting a 12 city tour might be the exact opposite of what a person who likes the hike through the mountains may want to do. Many people end up getting a standard tour, only to get �gtempled out�h after the 4th or 5th temple or shrine (with 18 or so planned on their month long trip).

My son did a river cruise along the Danube river, which included a city tour at each stop. Every tour included a tour of a local church. It went from �gWhoa, this cathedral is HUGE!�h to �gYup... It�fs another cathedral.�h after about a week.

Good luck, in planning your vacation.