Summary
If you make 20,100,000 円 a year living in Japan, you will be taxed 6,418,482 円. That means that your net pay will be 13,681,518 円 per year, or 1,140,127 円 per month. Your average tax rate is 31.9% and your marginal tax rate is 38.4%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of 100 円 in your salary will be taxed 38.42 円, hence, your net pay will only increase by 61.58 円.
Bonus Example
A 1,000 円 bonus will generate an extra 616 円 of net incomes. A 5,000 円 bonus will generate an extra 3,079 円 of net incomes.