A Chinese naval intelligence ship was spotted off the Boso Peninsula in Chiba Prefecture several times from Wednesday to Saturday, the Japanese Defense Ministry said.
The vessel did not intrude into Japanese territorial waters, the ministry said, but added it is rare for a Chinese naval vessel to sail back and forth in that area near Tokyo.
Japan has conveyed to China through a diplomatic channel its concern over the incident, and the Defense Ministry has begun to investigate why the Chinese naval intelligence ship was sailing there.
The vessel repeatedly sailed to the northeast and then turned toward the southwest outside the contiguous zone — the area within 12 nautical miles (22 km) of a nation's territorial waters — from around 3:30 p.m. Wednesday to around 11 a.m. Saturday, according to the Defense Ministry.
In November, a Chinese naval intelligence ship sailed back and forth near the Japan-controlled Senkaku Islands. China claims the islands and calls them the Diaoyu.