There definitely is not a 6-month or similar validity requirement, but I can't directly find proof of this from any "official" source either.
The closest I can find directly from the Japanese government is from a page aimed at Japanese citizens travelling abroad:
http://www.gov-online.go.jp/useful/a.../200807/3.html
世界の多くの国々では、日本と異なり、パスポートに一定以上の有効期間が残っていることを入国条件の一つと しています。
Unlike Japan, many countries require that a passport have at least a certain validity period remaining as one condition of entry.
This
page from the Ministry of Foreign Affairs lists the requirements for entry into Japan for citizens of countries that do not require a visa, and there is no mention of anything other than a "valid passport."
There are also a number of less-official sources confirming this:
http://oshiete.goo.ne.jp/qa/5768660.html
http://detail.chiebukuro.yahoo.co.jp...il/q1116241116