Parts of central and western Japan were rocked by a strong, magnitude 6.5 earthquake on Friday.
Local media and authorities reported some buildings had collapsed while people were injured in the quake centered off Japan's western prefecture of Ishikawa.
Chief Cabinet Secretary Hirokazu Matsuno told an emergency press conference that one person was in cardiac arrest after the quake, but gave no further details.
No abnormalities were reported at the Shika nuclear power plant in the area, or at the Kashiwazaki-Kariwa plant in the neighboring prefecture of Niigata, Matsuno added.
Japan's weather authorities revised the quake's magnitude to 6.5 from a preliminary 6.3, and warned against aftershocks, although it had triggered no tsunami warning.
"Earthquakes of large magnitude may occur, especially in the next three days," a meteorological agency official said, adding that tremors followed the large quake.
He urged residents of quake-hit areas to keep watch for about a week in case of further events of seismic intensity 6 or higher on the Japanese scale, which runs to 7.
The government has set up an earthquake response center headed by Prime Minister Fumio Kishida, Matsuno said.
"Prime Minister Kishida instructed us to do everything in our power for relief and rescue," he added.
"We will take whatever action is necessary, depending on the damage and impact from the earthquake," Matsuno said in reply to a query whether the quake would affect plans for Kishida to visit South Korea from Sunday.
