Disclosed is a disc insertion preventive device comprising a housing having a disc slot made on its front side, a generally stopper and a swinging mechanism. The stopper comprises a flat plate and front and rear blocking pieces dependent from the opposite lateral edges of the flat plate. The stopper is pivotally fixed to the housing with the front blocking piece directed toward the disc slot of the housing. The swinging mechanism is responsive to absence of a disc in the housing for inclining the stopper with the rear blocking piece down and with the front blocking piece up, thus preventing advance of a disc beyond the descendent rear blocking piece, and is responsive to presence of a disc in the housing for inclining the stopper with the rear blocking piece up and with the front blocking piece down, thus preventing insertion of another disc from the disc slot.