An information recording-reproducing apparatus includes carrier rollers arranged in a carriage path having a width almost equal to a diameter of a large diameter disc, and a mechanical side switch PSW at one peripheral end of the carriage path. First and second optical detectors are arranged in the carriage path at an interval smaller than a diameter of the small diameter disc. The first optical detector and the side switch are arranged at an interval larger than the diameter of the small diameter disc, while the second optical detector and the side switch are arranged at an interval smaller than the diameter of the small diameter disc. The type of the disc as well as difference between the disc and an object other than the disc are determined based on the detection signals from the optical detectors and the side switch.