The central bank's past efforts to spur growth have disappointed investors and there is expectation of more monetary easing from the meeting on Thursday.
Japan's Nikkei 225 index fell 0.5% to close at 17,353.28.
Embattled Mitsubishi Motors again saw its shares tumble, losing 9.6% over the day.
The Japanese car maker is engulfed in a scandal over falsifying fuel efficiency.
In China, the mainland benchmark Shanghai Composite closed 0.6% higher at 2,964.70 while in Hong Kong the Hang Seng ended up 0.5% at 21,407.27.
In South Korea, the Kospi index finished the day up 0.3% at 2,019.63, despite the country reporting its slowest quarter of economic growth in nine months.
In the January to March period the economy grew by 0.4% compared with the previous quarter, which was in line with expectations. On an annual basis, the economy expanded 2.7%.
In Australia, traders were equally cautious, and the ASX/200 index fell 0.3% to 5,220.64.
Japanese shares fell for a second day as investors remained cautious ahead of a key Bank of Japan meeting later this week.