There have been no reports of abnormalities at nuclear power plants following the quake, and the government is checking for damage and casualties, Chief Cabinet Secretary Yoshimasa Hayashi told reporters.
The quake occurred at around 4:43 p.m. (0743 GMT) off Miyazaki prefecture on the western major island of Kyushu, the JMA said.
In Miyazaki, waves as high as 50 cm (20 inches) have already been observed, public broadcaster NHK said.