Summary
If you make 20,478,468 円 a year living in Japan, you will be taxed 6,563,896 円. That means that your net pay will be 13,914,572 円 per year, or 1,159,548 円 per month. Your average tax rate is 32.1% 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 38.43 円, hence, your net pay will only increase by 61.57 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.