A 4,900 JPY monthly subscription fee is part of the standard Amazon Services Business Solutions Agreement to which you agreed when you created your Professional selling plan account. Please refer to the terms of your agreement for more information.
You will find more information about fees in the Selling on Amazon Fee Schedule.
If you would like to sell on Amazon without the monthly fee, you can switch your selling plan to Individual and instead pay a fixed per-item fee when the item sells.