The quake occurred in the East China Sea, 218 kilometres (136 miles) west of Naha, Okinawa prefecture, the US Geological Survey said. The quake's depth was 222 kilometres (138 miles).
There were no immediate reports of injuries or damage from the quake, which happened at 11:59 am (0259 GMT).
Japan, located at the junction of four tectonic plates, experiences 20 per cent of the strongest quakes recorded on Earth each year.
A devastating tsunami on March 11 left about 22,000 people dead or missing and triggered an atomic crisis at the Fukushima Daiichi nuclear plant in the northeast of the country.
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