Nikkei Dives More Than 6 Percent HONG KONG - The battered Japanese stock market lurched into bear market territory Thursday, after a tumble of 6.4 percent took the combined decline in the Nikkei 225 index since a peak on May 23 to more than 21 percent. A pronounced sell-off in the morning and early afternoon accelerated shortly before the markets closed in Tokyo. The Nikkei ended down more than 840 points at 12,445.38, its lowest level since early April. The drop on Thursday was one of many sharp declines seen in recent weeks, since a feverish six-month rally in Japanese stocks - incited by optimism over the government's aggressive efforts to reinvigorate the listless economy - came to an abrupt end. The Nikkei 225 soared more than 80 percent between mid-November and mid-May, but staged a sudden about-face with a 7.3 percent plunge on May 23. Sentiment has been fragile and trading volatile ever since, as investors have taken stock of the challenges that face "Abenomics," the economic policies of Prime Minister Shinzo Abe, and weighed the pros and cons of taking profits after the rally. In Tokyo, Yoshihide Suga, the chief cabinet secretary, brushed off the market plunge. "I feel it's important not to swing from joy to sorrow every time stock prices rise or fall, and keep on doing what we need to do," Mr. Suga said at a news conference Thursday morning, as shares fell. The Japanese economy is steadily improving.' Meanwhile, the Bank of Japan governor, Haruhiko Kuroda, met with Mr. Abe on Thursday to exchange views on the economy, according to local news reports. Mr. Abe told the governor he was determined to do his part in putting a growth plan into action, Mr. Kuroda later told reporters. Mr. Kuroda told the prime minister that the central bank was committed to supporting the Japanese economy through monetary stimulus. Mr. Kuroda also said he expected markets to soon "calm down to reflect positive developments in the economy," according to the Nikkei Web site. On Monday, the Japanese government revised up its first-quarter gross domestic product figures, saying its economy grew at an annualized pace of 4.1 percent between January and March, better than the 3.5 percent it initially reported. But that upgrade has not been enough to calm investor jitters. But factors beyond Japan also have come into the fray, and helped send markets lower around the world. In China, which is a key engine of global growth, the flow of economic data in recent weeks has reinforced the picture of an economy that is struggling to regain momentum. And in the United States, comments on May 22 by Ben S. Bernanke, the chairman of the U.S. Federal Reserve, that he and his colleagues might consider paring back their bond-buying programs "in the next few meetings" if the economy is showing signs of improvement have helped fan global nervousness. Investors and analysts, meanwhile, have struggled to assess the possible implications of even a small withdrawal of the bond buying that has supported markets in recent years. The concerns about the "tapering" of U.S. stimulus measures have sent stocks lower around the world. In the United States, the Dow Jones industrial average and the S. & P. 500 have sagged 3.2 percent and 4 percent, respectively, in the past three weeks. The DAX in Germany has fallen about 4.5 percent and the CAC 40 in France has dropped more than 6 percent. Key markets in the Asia-Pacific region have tumbled even more. The Straits Times index in Singapore and the Hang Seng in Hong Kong have both shed more than 10 percent since May 22, and in Australia, the S.&P./ASX 200 has sagged more than 9 percent. Hiroko Tabuchi contributed from Tokyo.