A wave of one metre (three feet) is expected to hit the coast of the Sea of Japan, north of Tokyo, the nation's meteorological agency said.
Tokyo:
Japan issued a tsunami advisory Tuesday after a strong 6.8-magnitude earthquake hit the northwest of the country.
Officials immediately stopped bullet train services in the region as a precautionary measure, according to public broadcaster NHK, which also said around 200 households were without power.
The meteorological agency said some waves were expected to have already reached some coastlines of Yamagata and Niigata, in the northwest of the country.
NHK said no abnormality was monitored at nuclear power plants near the epicentre.
The quake registered six on the Japanese scale, which goes up to seven.
Japan sits on the Pacific "Ring of Fire" where many of the world's earthquakes and volcanic eruptions are recorded.
Last June, a deadly tremor rocked the Osaka region, killing five people and injuring over 350.
On March 11, 2011, a devastating 9.0-magnitude quake struck under the Pacific Ocean, and the resulting tsunami caused widespread damage and claimed thousands of lives.