The Bank of Japan can double its annual pace of bond accumulation to ¥100 trillion to give fresh impetus to the economy after next month's sales tax increase, according to an aide to Prime Minister Shinzo Abe.

"It's not taboo for the BOJ to double the goal of bond holdings," Koichi Hamada, 78, a retired Yale University professor who advises Abe on monetary policy, said in a recent interview in Tokyo. "The BOJ will be concerned about the real risk of being seen as financing debt, but drastic action is justified to pull Japan's economy out of 15 years of stagnation."

Hamada said the central bank should add stimulus as soon as May should indicators show the 3 percentage point tax rise is seriously damaging the economy. He said annualized growth of 0.7 percent in the final quarter of 2013 showed that " 'Abenomics' isn't strong enough."