From February 1st 2008, all people entering Japan must present a "Declaration of personal effects and Unaccompanied articles" card to the customs inspector upon their arrival. This declaration card aims to help prevent terrorism and smuggling by various international criminal organizations, and also allows passengers to clear Japanese customs inspections smoothly.
For details, please refer to the Japanese Customs Website http://www.customs.go.jp/
You may also use & present A4 sized declaration form downloaded from below URL on the inspection of Japan Customs.
One declaration card per family is permitted.