To provide a keyhole illegal intrusion preventing device put in a double locked state compactly without impairing appearance with a short projection from the surface of a keyhole in view of the frequent occurrence of picking, lock breaking, and the like.

This keyhole illegal intrusion preventing device is constituted to identify the sides of hexagonal rings by colors to thin the width of the projection in order to shorten the projection to a minimum when mounted to the key hole. A compact lock can thereby be formed with a thickness of a ring lock 4 being about 11 mm. A hook 3 is pressed into the key hole, and a wedge 2 is inserted to spread out the hook 3, and a ring lock support 1 sticks closely to the keyhole to put the ring lock 4 into an unlocked state. The hexagonal rings 5, 6, 7 are turned to the ring lock support 1 to lock.