When a normal disk having the diameter of 12 cm is inserted into the disk device, first and second optical detecting elements detect the insertion, and transitions to a first detection state, which initiates transfer operation of first and second transfer rollers. If the disk is further moved to an inner portion of the device, thus pressing first and second detecting pins, first and second detecting switches are turned OFF, and the detecting elements enter a second detection state. Before the transition from the first detection state to the second detection state occurs, a rotating speed of the first and second transfer rollers is set low. After transition to the second detection state, the rotating speed is increased. As a result, a card or other such foreign objects can be removed and a monitoring period can be increased until detection of a normal disk is detected.