Moving to Tokyo / Suitable salary

Hello,
you'll see that the costs of living in Tokyo can be very different depending on where you live and how you live (i.e. lifestyle related).
Bear in mind that you've been offered a Gross Salary, at 5.1 M.JPY it means in Net:
income tax @ 10% - 97.500 deduction = 412.5000 JPY per year for income tax which is deducted directly by your employer.
This will give you a base Gross Salary of 390.625 JPY per month
then you'll have about 12% deduction for social security (retirement, unemployment and health insurance) this will leave you with about 343.750 JPY net per month. This is pretty good considering your age. You should not have too much trouble living if you follow the golden rule of the 3 thirds, 1/3 in rent, 1/3 for living, 1/3 saved. Saved for the Ward and Prefectural Tax which come the following year (at 6% and 4% each respectively).
PM me if you want more tips.