Some investors are worried about the market impact when the Bank of Japan decides to wind back an extraordinary yearslong program of support for the country's stock market, but the world's largest money manager sees less reason for concern.

There will probably be some initial volatility for equities when the BOJ announces a slowdown in purchases of exchange-traded funds, according to Jason Miller, the head of BlackRock Inc.'s iShares business in Tokyo. But the move may then be interpreted as a sign of confidence in the economy and market, he said.

The central bank has been aiming to spend about ¥6 trillion ($54 billion) a year on ETFs since it doubled its purchase target in 2016 as it stepped up a massive stimulus program. Some have speculated that BOJ Gov. Haruhiko Kuroda and his team are tacitly moving away from the program, as monthly purchases of ETFs tumbled in July and August. The bank has denied it.