Summary
If you make 1,299,996 円 a year living in Japan, you will be taxed 243,638 円. That means that your net pay will be 1,056,358 円 per year, or 88,030 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.