* Nikkei stumbles to lowest since September 2017
* Topix slips to lowest since April 2017
* U.S. futures' weakness exacerbates mood
* SoftBank Corp continues to weaken after debut
* BOJ keeps policy steady as expected
By Ayai Tomisawa and Shinichi Saoshiro
TOKYO, Dec 20 (Reuters) - Japan's Nikkei skidded to a 15-month low on Thursday after the U.S. Federal Reserve largely retained its plans to increase interest rates next year despite rising risks to growth, triggering a sell-off in global stocks.
The Nikkei share average tumbled 2.8 percent to 20,392.58, the weakest closing point since September 2017.
The broader Topix declined 2.5 percent to 1,517.16, the lowest closing level since April 2017.
Following a two-day policy meeting the Fed raised interest rates on Wednesday for the fourth time this year as widely expected.
The U.S. central bank also forecast fewer interest-rate increases in 2019, but it's core plan to keep tightening rates even as global growth slowed spooked investors, sending Wall Street shares tumbling.
"Most investors wanted to wait for the Fed's decision to take positions, and now global investors are shifting to safe haven assets from stocks," said Hiroyuki Ueno, a senior strategist at Sumitomo Mitsui Trust Asset Management.
Ueno said that investors are becoming risk-averse on worries about a slowdown in the global economy, which is reflected in a sell-off in cyclical stocks such as machinery, shippers and tech shares.
Mitsui OSK Lines nosedived 6.9 percent and Nippon Yusen declined 4.6 percent, while Panasonic Corp stumbled 5.1 percent, Fanuc Corp shed 3.6 percent and Tokyo Electron slumped 4.3 percent.
Selling accelerated in the afternoon when U.S. futures fell, pointing to another weaker start on Wall Street later in the day.
In Asian afternoon trade, Dow e-minis slipped 0.6 percent, S&P 500 e-minis dropped 0.8 percent, and Nasdaq 100 e-minis declined 0.7 percent.
Shares of telecoms operator SoftBank Corp remained volatile after tumbling 15 percent on its debut the previous day. The slide inflicted big losses on retail investors who bought into the household name in Japan's biggest ever IPO.
SoftBank Corp shared ended a roller-coaster session down 4.7 percent.
Takeda Pharmaceutical Co jumped 7 percent after it announced the listing of American depository shares on the New York Stock Exchange later this month.
Takeda had sunk to a six-year low of 3,498 yen on Wednesday after a ratings downgrade by Moody's.
Taisho Pharmaceutical Holdings Co climbed 2.4 percent following news that it offered to buy French over-the-counter drugs business UPSA for $1.6 billion from Bristol-Myers Squibb Co.
On Thursday, the Bank of Japan kept monetary policy steady as expected, which markets took in their stride. It stuck to its optimistic view on the economy, even as simmering Sino-U.S. trade tensions and volatile financial markets cloud the outlook for global growth. (Editing by Shri Navaratnam)