If you have lived in Japan for less than 5 years in the last 10, then you will be categorized as a non-permanent resident for tax purposes.
This means that you will be taxed on your domestic sources of income. However, if you remit into Japan any foreign earnings, those will be viewed as remittance income and you will have to declare them.
Additionally, if you bring in money in bulk, that will also have to be declared as remittance income.
For more information or for the preparation of your income tax return, please feel free to contact us.