Summary
If you make 8,400,000 円 a year living in Japan, you will be taxed 2,450,886 円. That means that your net pay will be 5,949,114 円 per year, or 495,760 円 per month. Your average tax rate is 29.2% and your marginal tax rate is 29.2%. 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 29.21 円, hence, your net pay will only increase by 70.79 円.
Bonus Example
A 1,000 円 bonus will generate an extra 708 円 of net incomes. A 5,000 円 bonus will generate an extra 3,540 円 of net incomes.