Summary
If you make 1,221,420 円 a year living in Japan, you will be taxed 228,911 円. That means that your net pay will be 992,509 円 per year, or 82,709 円 per month. Your average tax rate is 18.7% and your marginal tax rate is 18.7%. 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 18.74 円, hence, your net pay will only increase by 81.26 円.
Bonus Example
A 1,000 円 bonus will generate an extra 813 円 of net incomes. A 5,000 円 bonus will generate an extra 4,063 円 of net incomes.