The invention provides a jam-proof device of a slot-in optical disk drive. In the jam-proof device, a contact pin protrudes from one end of the loading bar, the other end of the loading bar is pivoted with a pivot on a first side of the casing, and a block protrudes adjacent to the pivot. One end of the unloading bar is pivoted on a second side of the casing opposite to the first side, and a reception end is formed at the other end of the unloading bar to extend to a center of the casing. A distance between the block and the reception end at a default position is shorter than a diameter of an 8-cm disc to prevent the disc from entering the slot-in optical disk drive.