The tremor struck at 6:18 am (2118 GMT) with its epicentre 800 kilometres (496 miles) south of Tokyo, close to the Izu islands, the Japan Meteorological Agency said.
"You don't have to worry about disastrous tsunamis that could lead to damages, as certain time has passed since the quake," an official at the agency told AFP about two hours after the tremor.
He added that the agency will keep monitoring the sea level and that "some sea-level changes may be observed due to this quake."
Japan lies at the junction of four tectonic plates and endures about 20 percent of the world's most powerful earthquakes, which frequently jolt Tokyo and other major cities.
On Friday two strong undersea earthquakes struck Japan but with no reports of damages of casualties.

Copyright AFP (Agence France-Presse), 2006