A library device in which, even if the relation between the number of pulses of a stepping motor and an encoder count number changes, a movement distance of a medium transfer means can be verified by an encoder count number counted by an encoder sensor is provided.A control means (2) determines a proof factor used when calculating an assumed encoder count number assumed based on the number of pulses according to a movement distance of a medium transfer means (5). When a movement distance of the medium transfer means (5) is verified, an assumed encoder count number, which is assumed based on the above-mentioned determined proof factor and the number of pulses according to the movement distance of the medium transfer means (5), and an actual encoder count number, which is counted by an encoder sensor when a stepping motor is driven by the number of pulses according to the movement distance of medium transfer means (5), are compared.