A Chinese ship was first spotted entering the Tsugaru Strait near Cape Tappi in the northern Aomori prefecture in the early hours of Friday. Japanese coast guards issued several radio warnings before the ship left the area.
More Chinese vessels entered Japanese waters several hours later and stayed there for about 90 minutes.
This is the first time in two years that Chinese ships entered the strait between the Honshu and Hokkaido islands.
They are more often seen sailing near what China calls its Diaoyu Islands, in the East China Sea further south. The uninhabited islands are also claimed by Japan, which calls them Senkaku.