TOKYO—Japan's top central banker took the unusual step of weighing in on the country's parliamentary election campaign, criticizing the platform of the front-runner for prime minister as "unrealistic" and "negative" for the economy.

Bank of Japan Gov. Masaaki Shirakawa on Tuesday rebutted point by point the detailed monetary-policy proposals offered in recent days by Shinzo Abe, head of the opposition Liberal Democratic Party, which currently leads in all major public-opinion polls...