* BOJ to expand types of assets it buys
* But BOJ will not expand base money target
* Dollar/yen rises briefly, then slips (Updates with dollar/yen reaction to BOJ)
By Shinichi Saoshiro and Ian Chua
TOKYO/SYDNEY, Dec 18 (Reuters) - The dollar slipped against the yen on Friday after a set of stimulus measures the Bank of Japan unveiled were deemed too modest to drive the greenback higher.
The BOJ on Friday said after its policy meeting that it would expand the types of assets it purchases, sending the dollar briefly soaring versus the yen.
But the greenback slid as it became clear that the central bank would not expand its base money target under its massive stimulus programme.
The dollar initially rose to 123.59 after the BOJ announcement, but was last down 0.3 percent at 122.145 yen.
The BOJ was widely expected to stand pat on monetary policy and the market was caught flat-footed.
"I don't think many people were expecting the BOJ to make a move right after the FOMC, so the timing of the announcement came as a surprise," said Ayako Sera, senior market economist at Sumitomo Mitsui Trust in Tokyo.
"But upon closer inspection the contents show that the BOJ merely fine-tuned its policy, which is why the dollar came down."
The central bank kept intact its policy target of increasing base money - or cash and deposits in circulation - at an annual pace of 80 trillion yen ($655 billion) via aggressive buying of government bonds, exchange-traded funds (ETFs) and trust funds investing in property.
In addition to such purchases, the BOJ decided to set aside 300 billion yen to buy ETFs that specifically target shares of companies actively pursuing capital expenditure. It will also extend the maturity of JGBs it buys to 12 years from 10 years next year.
The U.S. dollar index slipped 0.4 percent to 98.910 after hitting 99.294 overnight, its highest in two weeks. The index posted a 1.2 percent gain on Thursday - its biggest rise in over a month. The greenback advanced broadly after the Fed on Wednesday by lifting its benchmark rate off zero.
The euro was up 0.2 percent at $1.0846, pulling away from a 10-day low of $1.0802 hit overnight.
(Editing by Richard Borsuk)