Japan's Self Defense Force lost sensitive data pertaining to a joint US-Japan military exercise last year, the Ministry of Defense said Tuesday.

The loss occurred just before an apparently more serious case in which information regarding the Aegis missile system was found on the home computer of a Self Defense Force member, and could bring further criticism from the US of Japan's military and its data handling.

The case detailed Tuesday concerns data on a joint training exercise that was stored on a USB stick. The stick was taken by a captain in the Ground Self-Defense Force who later threw it in the trash, Defense Minister Shigeru Ishiba said at a news conference.

The data concerned unit deployment maps, in particular those of US helicopters and tanks, and was categorized as requiring caution in its handling but it was not confidential, the Mainichi Shimbun reported in its Tuesday morning edition. Ishiba was speaking at the news conference in response to the newspaper's report.

The Aegis data incident, which came to light in March last year, resulted in the arrest of an officer of the Maritime Self Defense Force and embarrassed Japan at a time when it was trying to persuade the US government to give it some access to information regarding the F22A Raptor aircraft ahead of a possible purchase. The Raptor is one of the most technically advanced in the US fleet and the leak dented confidence in Japan's ability to keep the information secret.

Japan apologized to the US regarding the Aegis incident last year, but as recently as last month the issue was still in the thoughts of the US government when Thomas Schieffer, US ambassador to Japan, speaking on defense cooperation, told a news conference, "The United States could do more if Japan increased its ability to protect classified material and proprietary information."