The Nikkei index at the Tokyo Stock Exchange lost 0.33 percent to end the early session at 9,736.07. The Topix index of all first section shares was flat, down 0.03 points to 853.10.
Tokyo shares were weighed by profit-taking after the Nikkei's 1.85 percent rise Friday, and as data showed a sharper-than-expected 2.3 percent on-month fall in Japan's February core machinery orders.
Such weakness before the devastating impact of the March 11 earthquake and tsunami on the world's third biggest economy indicated bleak data ahead, said dealers.
"The market is feeling quite nervous. If February is this bad, post-quake March will be even more severe," said Okasan Securities strategist Hideyuki Ishiguro.
He added that the ruling Democratic Party of Japan's defeat in several gubernatorial elections had little impact since Prime Minister Naoto Kan was unlikely to be replaced while the government was focused on reconstruction.
Exporters were generally lower with Sony down 2.18 percent and Toyota Motor down 2.10 percent.
Tokyo Electric Power, the owner of the Fukushima Daiichi nuclear plant that was battered by the quake, surged 13.10 percent after the Tokyo Stock Exchange imposed a temporarily higher margin-trading rate requirement for new investors.
Toshiba, which might decommission the plant, rose 4.15 percent.