An employee of the Osaka prefectural board of education planted a covert listening device in mid-June at its office in Chuo Ward, the board said Friday.
The bug was found Thursday by a TV crew that was shooting video footage for a program on wiretapping crimes.
On Friday evening, a male employee in his 40s confessed to his supervisor that he planted the bug, according to the board of education.
The man reportedly told his seniors he had "wanted to check what my colleagues say in their conversations." Then he set up a receiver and eavesdropped on about 20 occasions between mid-June and early August, including during his time at work, according to the board.
The TV crew, including a wiretapping expert, inadvertently picked up a signal from the bug, board staffer Makoto Taniguchi said by phone.
The device, hidden in an extension plug, was in the middle of four desks in the guidance section of the board's high school division.
Taniguchi quoted the wiretapping expert on the TV crew as saying the device can pick up voices from up to 8 meters away and transmit a radio signal up to 100 meters.
Such wiretapping devices are widely available at security-goods shops in the Nipponbashi electric district in Osaka and Tokyo's Akihabara district, where other items such as stun guns are also available.
In 1982, the man was employed by the Osaka Prefectural Government as a public school teacher and had worked for the prefectural board of education since 2000, the board said.
