Moving in with partner, changing address?

I'm currently terminating my contract with a house because of financial problems so I'm planning to move into my partner's apartment, who is a Japanese resident.

I was wondering how changing addresses will work? I know that when you move out or move houses, you need to get it changed on your Residence Card at the ward office. But will they allow it if the house isn't under my name? Will I be able to change address without any problems?

Will I be able to change address without any problems?
Yes.

People moving in with a boyfriend/girlfriend is pretty common in Japan, so registering your new address probably won't be a problem. The bigger issue is likely to be between your partner and his landlord, as lease agreements in Japan often specify the number/identity of apartment residents. There's a chance your partner's lease limits his apartment to a single occupant, and so he might need to consult with his landlord if you're going to be living with him long-term.

I see! Thanks so much for the replies. I figured that'd be the problem but I'll make sure to ask the landlord beforehand. But when you change addresses at the ward office, how do they make sure I'm not just registering a random address? Will they ask for proof?

I don�ft think that they ask for proof.

My cousin was living with me in Japan for a year or so, and I think he just went to the ward office and gave our address and got registered. I don�ft remember any special paper work he needed and definitely nothing from the landlord.

Just confirm if it is allowed by the landlord to move together.
I do not know if you move within the same city or not, but
if the landlord allows the move in than go to the city hall and inform your movement.
You will receive a documents from the city hall if you move out the city and this document you give to the new city hall and inform them the new address.

If required you need to inform the immigration as well.

Those are simple steps and you do not need to be worried about it

They don't normally ask for proof, but be careful because if you make a false declaration (register an address where you don't actually live) and immigration ever finds out, the penalties can be very stiff, up to and including imprisonment and deportation.

But when you change addresses at the ward office, how do they make sure I'm not just registering a random address? Will they ask for proof?
No proof required. But if someone did register a random address it would turn badly quite quickly as mail sent to your official address would include things like resident tax. Not getting your tax bills and failing to pay them would quickly cause you issues - especially at visa renewal time.

Thank you for the replies! It really helped out. Oh wow, that's insane. But I will be living there so it should be fine.

I do have one last question though, I was away for awhile and couldn't keep up with my insurance bills when I got back. And now my insurance has expired? I think. How do I fix this? Should I do this when I change my address or should it be solved immediately?

Sorry for asking so many questions

Which insurance?
If you have not paid your insurance you need to contact the company.
Inform them and make an arrangement, because you can be put on a list for not paying which gives you problems in the future.

It's my Houkensho which is my health insurance. I was thinking if it's possible if I could contact them and explain my situation. But should I go to the ward office and renew it?

please read justmyday's post:
"You will receive a documents from the city hall if you move out the city and this document you give to the new city hall and inform them the new address."

you go to (present and new) city offices (i.e. two places) and follow their directions, without omitting. foreigners who will have troubles mostly come from that they don't follow their directions.
bring your health insurance card, your zairyu card, pension book, and passport, for the case that they want to see them.

arguments about landlord is not your concerning point. that is your partner's problem.