How to ship big package from Japan to USA

Is shipping a large box size package from Japan to USA easy like USA to Japan?

How big are you talking about? The boxes you can send from post office have some size restrictions, so that's why I'm asking... if it's not too big, you simply take it to the post office, ask them about the postage (air, surface, etc.), fill out the customs form, and have it sent.

So it is easy to send items to USA,
like it is easy to send items from USA to Japan, is what I'm getting. Thanks

If it is within the postal weight limits (maximum 30 kg), you can use the excellent English postage calculator here to estimate the charges:
http://www.post.japanpost.jp/cgi-charge/index.php?lang=_en

There are several price options, depending how fast you want it to arrive (obviously, slower is cheaper).

Note that there are also U.S. customs regulations about what you are permitted to send to the U.S., and the maximum value per package (or gift recipient) without paying extra duty (paid in the U.S., not at the post office in Japan). You should look at the U.S. customs page for details about that.

Is the package heavy as well as large? There is a very good value if the package weighs less than 2 kg, but as you describe it as "large," that probably doesn't apply.