Japan's key indicator of the money supply grew at an average pace of 2.2 percent in May from a year earlier, a slowdown from the 2.9 percent gain in April, the Bank of Japan said in a preliminary report released Monday.

The balance of M2 -- currency in circulation and demand and time deposits -- plus certificates of deposit, averaged 630.5 trillion yen per day, the central bank said.

The rate of growth accelerated in April for the first time in six months.

The balance of M2 plus CDs -- held principally by firms, individuals and local governments -- is considered to be a key money supply gauge correlating to changes in monetary policy options.

The BOJ said currency in circulation grew by 7.4 percent, down from a revised 9.8 percent in April.

Growth in money in demand deposits and checking accounts slowed to 11.1 percent from April's 12.4 percent.

The balance of quasimoney -- money in time deposits, deferred savings, yen deposits by nonresidents and foreign-currency deposits -- contracted 1.4 percent, narrower than the 1.6 percent shrinkage in April.

The balance of CDs decreased 15.2 percent, an acceleration from the 5.8 percent fall in April.

Broadly defined liquidity -- the widest measure of the money supply -- increased 3.1 percent to 1.27 quadrillion yen in May, marginally slower than the revised 3.2 percent rise in the previous month.

The gauge includes postal savings, deposits at agricultural and fishery credit cooperatives, loan trusts and money trusts, bank debentures and government bonds, on top of M2 plus CDs.