Putin's health blamed for delayed state visit
"It's about (President Putin's) health problem. This is not something that can easily be made public," Jiji news agency reported an official as saying, quoting Mr Noda.
Mr Putin's health has been a subject of media speculation since he was seen limping at an Asia-Pacific summit in the Russian port of Vladivostok in September.
The Russian government revealed in October that Mr Putin, who turned 60 last month, was suffering from back trouble, but the Kremlin denied it was a serious problem.
Belarussian President Alexander Lukashenko linked the problem to judo. "He lifted a guy, threw him, and twisted his spine," he said.