A library device, comprising: a medium storing means that can accommodate a plurality of computer-readable media; a medium reading and writing means for performing at least one of reading information stored in said medium and writing information in said medium; a medium transfer means including a stepping motor and an encoder, for transferring said medium between said medium storing means and said medium reading and writing means; a memory where a plurality of adjustment factors are stored, said adjustment factors being preliminary determined by dividing a predetermined number of pulses of the stepping motor by respective actual count numbers counted by the encoder; and a control means for controlling said medium transfer means, wherein said control means includes: a movement control means for moving said medium transfer means by driving a stepping motor by a number of pulses according to a movement distance of said medium transfer means; an adjustment means for determining which one among said adjustment factors is to be used as a proof factor at a time of calculating an assumed encoder count number assumed based on said number of pulses; and a verifying means for comparing an actual encoder count number counted, upon said stepping motor having been driven by said number of pulses, by an encoder sensor according to a movement of said medium transfer means, and said assumed encoder count number assumed based on said number of pulses and of the proof factor determined by the adjustment means. a medium storing means that can accommodate a plurality of computer-readable media; a medium reading and writing means for performing at least one of reading information stored in said medium and writing information in said medium; a medium transfer means including a stepping motor and an encoder, for transferring said medium between said medium storing means and said medium reading and writing means; a memory where a plurality of adjustment factors are stored, said adjustment factors being preliminary determined by dividing a predetermined number of pulses of the stepping motor by respective actual count numbers counted by the encoder; and a control means for controlling said medium transfer means, wherein said control means includes: a movement control means for moving said medium transfer means by driving a stepping motor by a number of pulses according to a movement distance of said medium transfer means; an adjustment means for determining which one among said adjustment factors is to be used as a proof factor at a time of calculating an assumed encoder count number assumed based on said number of pulses; and a verifying means for comparing an actual encoder count number counted, upon said stepping motor having been driven by said number of pulses, by an encoder sensor according to a movement of said medium transfer means, and said assumed encoder count number assumed based on said number of pulses and of the proof factor determined by the adjustment means. a movement control means for moving said medium transfer means by driving a stepping motor by a number of pulses according to a movement distance of said medium transfer means; an adjustment means for determining which one among said adjustment factors is to be used as a proof factor at a time of calculating an assumed encoder count number assumed based on said number of pulses; and a verifying means for comparing an actual encoder count number counted, upon said stepping motor having been driven by said number of pulses, by an encoder sensor according to a movement of said medium transfer means, and said assumed encoder count number assumed based on said number of pulses and of the proof factor determined by the adjustment means.