To surely detect a boarding ticket or a card or paper sheets or the like.

This medium detecting device is provided with a conveyance path for conveying a medium inputted to an input port, a light emitter arranged in the middle of the conveyance path and at one side with the conveyance path as a middle for emitting a light beam to the conveyance path, a reflector arranged at the other side with the conveyance path as a middle for reflecting the light beam emitted from the light emitter to the conveyance path side, a light receiver arranged at one side with the conveyance path as a middle so that a predetermined interval can be maintained with the light receiver for receiving the light beam reflected by the reflector and a deciding means for deciding that the medium is not positioned on the conveyance path when the light beam emitted from the light emitter is received by the light receiver, and for deciding that the medium is positioned on the conveyance path when the light beam emitted from the light emitter is not received by the light receiver.