To provide an information record medium locking device for preventing theft of an information record medium, capable of using an existing information record medium as it is by providing a locking means constituted, so as to be selectively projected or withdrawn in an insertion and discharge path for taking in and out the information record medium.

To a main body device 1, a card 2 which is an information record medium is inserted from an insertion and discharge path 3. Also, inside the main body device 1, a clasp which is a locking mechanism is present. A key is provided as the one for operating the clasp from the outside. When the key is used, a key hole 4 matched with the clasp is provided in the main body device 1. Then, by rotating the key in a discharged state to the key hole 4, the clasp is switched by being positioned in the insertion and discharge path 3 of the card 2 or being withdrawn from the insertion and discharge path 3. As a result, theft of the card 2 is prevented, without the rod for providing special mechanism such as a notched part or the like.