The quake hit at 11:39 am (0239 GMT) off the coast of Japan's main Honshu island, at a location about 350 kilometres (395 miles) southwest of Tokyo.
The quake hit at a depth of 10 km just before midday local time and was felt throughout Wakayama prefecture.
Local media reported the quake initially at magnitude 6.1 with NHK saying Tokaido Shinkansen services were suspended temporarily as a precaution.
No tsunami alert was issued. Although there may be slight sea-level changes in coastal regions, no damage was expected, the Japan Meteorological Agency said
Earthquakes are common throughout Japan; more than 750 have been registered in the past year.
Japan sits at the junction of four tectonic plates and experiences around 20 percent of the world's most powerful earthquakes.
But rigid building codes and strict enforcement mean even powerful tremors frequently do little damage.
A massive undersea quake that hit in March 2011 sent a tsunami barrelling into Japan's northeast coast, leaving 18,500 people dead or missing, and sending several reactors into meltdown at the Fukushima nuclear plant in the worst atomic accident in a generation.
CNA/Reuters/AFP/jb
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