Basically Japanese income tax rates for non-residents is 20% *, however, if you are a resident of any of the countries which have a tax convention with Japan, you may be eligible for tax relief.
»See "Income Tax Convention Country List"
The provision is different depending on each tax convention, so we recommend you to contact your legal and/or tax advisor if you have any question.
References:
Special measures provided by international tax treaties (National Tax Agency JAPAN)
http://www.nta.go.jp/tetsuzuki/shinkoku/shotoku/tebiki2011/pdf/43.pdf
(Please refer to the list of countries on page 64)
*[NOTE] Special Income Tax for Reconstruction in force from 2013
The tax rate has been increased due to "special income tax for reconstruction", which is focusing on measures for recovery and reconstruction from the Great East Japan Earthquake in 2011. The regulation is in force from Jan 1, 2013 to Dec 31, 2037 where the tax rate of 2.1% will be imposed on the standard National Income Tax.