To make the shape of parts simple, the transmission changeover well timed and reliable, and the size reduce, a transmission changeover mechanism in a disc apparatus includes a rotary control piece which is spring-biased to rotate in one direction, and has a transmission switching gear wheel fixed thereto. A retainer post stands on the chassis prevents rotation of the rotary control piece. A catch pin stands the rotary control piece. A cam rib is formed on lower surface of the traverse rack in confronting relation with the rotary control plate which is so configured as to make the rotary control piece release from the retainer post, and at the same time make the catch pin follow the rib contour, thereby allowing the transmission switching gear wheel to couple or decouple from the tray feeding gear wheel.