If you make 18,000,000 円 a year living in Japan, you will be taxed 6,073,446 円. That means that your net pay will be 11,926,554 円 per year, or 993,879 円 per month. Your average tax rate is 33.7% and your marginal tax rate is 39.1%. 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 39.14 円, hence, your net pay will only increase by 60.86 円.
A 1,000 円 bonus will generate an extra 609 円 of net incomes. A 5,000 円 bonus will generate an extra 3,043 円 of net incomes.