A Bank of Japan board member expressed pessimism about the economy and the central bank's strategy, saying in a speech Thursday that the BOJ will be unable to reach its 2 percent inflation target as forecast and negative rates will not work to boost investment.

"I believe that it is desirable to aim to achieve the price stability target of 2 percent as a medium-to long-term goal and I expect that the road toward this goal will be long," Takehiro Sato, one of nine BOJ board members, said in a speech in Kushiro, Hokkaido on Thursday.

Sato dissented on the January decision to adopt a negative-rate policy and also in October 2014, when the BOJ expanded stimulus.