Japan Coast Guard(JCG) is responsible for safety of Japanese seas. Similar to US Coast Guard, JCG sends its vessels and aircrafts for rescues and also check the vessels which are assumed to carry illegal stuffs.
Then, another important mission is to check unidentified vessels which are cruising on territorial seas and Japanese EEZ; exclusive economic zone. Unidentified vessels are assumed to be from North Korea, which has done multiple kidnapping in Japan and abducted dozens of innocent Japanese.
A battle of Amami-Oshima, commonly knowns as Spy Ship Incident in the Southwest Sea of Kyushu, has take place on 22 December 2001. It was a 6 hours confrontation between JCG and an armed North Korean vessel. Finally the vessel was sunk by JCG - JCG's first battle in the history and the vessel was later salvaged by JCG. Japanese government believes that it was a North Korean spy ship but of course North Korea does not admit.
The spy ship is currently can be seen at JCG museum Yokohama and there is no admission. Also photographing and video recording are permitted.