Japan's government on Thursday suspended a senior bureaucrat who blogged about the pointlessness of rebuilding tsunami-wrecked towns because they were populated mostly by economically unproductive old people.

The 51-year-old career civil servant said coastal areas that were destroyed in March 2011 were "almost in ruins" even before the disaster, which claimed more than 18,000 lives and turned whole communities to matchwood.

Writing anonymously, Hisanori Goto said it was unacceptable for public money to be spent rebuilding ports so that "old coots and hags can live off their vested interests in fisheries".

"Politicians who won't come out with the fair argument that reconstruction is unnecessary might as well die," he wrote.

He also captioned a picture of an elderly woman with the words "Drop dead".

Goto was unmasked after fellow bloggers were outraged by his comments and set about tracking him down, turning up a photograph of him that he had also posted on the web.

Chief Cabinet Secretary Yoshihide Suga said Goto had been confronted by bosses at the Ministry of Economy, Trade and Industry about the "inappropriate" comments, which he admitted writing in September 2011.

"It was an unforgivable act for a national public servant. We think it was extremely regrettable."

Jiji Press news agency later reported he had been suspended for two months.

Finance minister and former prime minister Taro Aso got into hot water earlier this year for saying elderly people should "hurry up and die" to avoid taxing the country's medical system.

He also faced international condemnation after he said Tokyo could learn from the Nazis' swift overhaul of Germany's constitution in the run-up to World War II.

Ill-considered words about the earthquake-tsunami of 2011, which also sparked the worst nuclear accident in a generation at Fukushima, are greeted with opprobrium in Japan.

Mainstream opinion holds that the ageing towns and cities on the northeast coast need to be rebuilt, despite their economic malaise and their quickening depopulation.