Seems very risky to me. There are many hidden traps on that road.
- Who pays relocation expenses? What benefits are included?
- How reliable is your income? Is it "occasional days", or "day after day with no breaks"?
You want to avoid situation where you paid for relocation, and you don't have steady income to rely on.
I hope I would never be in so desperate situation I would have to even consider such risky offer. Or vice versa, I hope the offer you have is so lucrative that taking such huge risk makes sense. Your situation may vary, and only you know if it is worth for you.
Offer might make sense for someone already located in Japan and lacking full time job, but otherwise? No thanks.
Edit: One more thing: unless you are very interested in relocating to Japan (which is not obvious from your question) and your skills are in such demand what you would be able to find another position very quickly, I would not consider relocating into country (with all the language and cultural barriers). and more: in contract, I would like to have a clause that if job will be cancelled in certain time (say under 6 months), I would want to receive severance pay which will allow me to relocate back. In writing.
Summary: thanks but no thanks for such offer. Japan if interesting country, but this is NOT interesting offer.