The Bank of Japan, along with the European Central Bank and the U.S. Federal Reserve, is taking unconventional steps to boost the economy. The BoJ boosted asset purchases by 10 trillion yen on February 14 and pledged to keep ultra-easy policy until a 1 percent inflation goal is in sight.
Speaking at a meeting of the Group of 20, Shirakawa said policymakers were closely watching the effects and side-effects of monetary easing on crude prices, which rose above $125 a barrel to end near a 10-month high on Friday.
But he said he did not see monetary easing as a big factor and the recent spike was more due to geopolitical tensions. Brent's recent gains have been fueled mainly by worries over Iranian supply.
(Reporting by Tetsushi Kajimoto, writing by Krista Hughes)