A library apparatus includes a cartridge type sensor and a robot. The cartridge type sensor includes a cartridge having the same shape as that of each recording medium stored in a cell, an optical reader for reading a medium identification code, and a medium detector for detecting the presence or absence of a recording medium in a target cell. The optical reader and the medium detector are disposed in the cartridge of the cartridge type sensor. The robot picks up the cartridge type sensor from a cell which stores the cartridge type sensor and positions the cartridge type sensor at a predetermined location in the library apparatus while holding the cartridge type sensor.