Those are some really general questions, Tsukasa, and to give you a better answer, we'd probably need to know more specifics beforehand. I would encourage you to check out some of the other threads in this forum related to living arrangements in Japan as a good starting point. Generally speaking, it really depends on the following:
A) Determine where exactly you are planning on living. I say this because the price of living arrangements can (and does) vary considerably depending on where you are (Tokyo vs Asahikawa, for instance).
B) How much money will you be taking with you beforehand to cover your initial deposit, shikikin ("thank you money"), reikin ("key money"), etc.
C) How much will you be earning monthly once your current employment starts (to determine whether or not you can cover the rent).
D) Considering most apartments don't come pre-furnished, it's a safe assumption that you will probably need to have enough money to bring with you to cover whatever basics you may need (appliances, futon, etc., just to name a few).
These are just a few steps one must consider, and as you can see, it is by no means an easy formula to figure as rates vary and fluctuate all the time. At any rate, when you have more specifics in mind--by all means, please feel free to post here, and we'll try our best to answer your questions.