Investigators have ruled out foul play because the room was undisturbed, and they were downplaying the likelihood of suicide. Determining a cause of death will likely "take some time," the spokesman said, adding that an autopsy will be conducted as part of an investigation.
The 56-year-old Nakagawa caused an uproar when he appeared to be intoxicated at a news conference during a meeting of Group of Seven financial leaders in Rome in February. International news programs repeatedly played footage of him slurring his speech and looking sleepy.
More odd behavior followed when he visited a museum at the Vatican after the news conference. He touched exhibits and set off an alarm after entering an off-limits area.
The trip was widely seen as a major embarrassment for the Japanese government.
Nakagawa stepped down as finance minister shortly afterward, denying he had been drunk and blaming cold medicine. But the opposition demanded his resignation.
Nakagawa had been a longtime lawmaker from the northernmost island of Hokkaido with the Liberal Democratic Party, which had ruled Japan almost continuously for the last half-century. He lost his seat in parliament in Aug. 30 nationwide elections in which the Liberal Democrats lost to the Democrats, who now rule Japan in a coalition.
Stunned colleagues said Sunday that Nakagawa appeared to be in good health recently but speculated that he may have been physically and mentally drained after losing his seat.
Former Prime Minister Taro Aso praised Nakagawa for helping the country tackle its worst recession since World War II.
"I'm in such a state of shock right now that I do not have words," Aso said. "I offer my deepest condolences."
Nakagawa had once been a rising star in the party, seen as a potential prime minister candidate. He held several Cabinet-level positions including agriculture minister and trade minister before being chosen as finance minister in September 2008.
His career - and untimely death - followed in the footsteps of his father Ichiro Nakagawa, who served in parliament for two decades. His father committed suicide in 1983 at a hotel in Hokkaido at age 57.
Nakagawa "had strong policymaking abilities," said Takeo Kawamura, who served as top government spokesman under Aso. "He would have played an important part in Japan's future political landscape."
Associated Press writer Yuri Kageyama contributed to this report.