To prevent the damage to a door lock device by forming an arcuate surface to a latch.

This failure-proof device is composed of the recessed section 2 of a body 1 made of a synthetic resin, a striker penetrating groove 4 formed to the body 1, a latch 5 pivoted into the recessed section 2, a striker 3 being engaged with the latch 5 and forward rotating the latch 5 at the time of the opening of a door, a half-latch engaging stepped section 7 and a full-latch engaging stepped section 8 formed to the outer circumferential surface of the latch 5, and a ratchet 9 being selectively engaged with the engaging stepped section 7 or the engaging stepped section 8 at the time of the closing of the door and preventing the reverse rotation of the latch 5. When the latch 5 is turned under the state of full-latch under the state, in which the door is opened without closing the door, the shape of an abutting section 14, with which the striker 3 is abutted, and the side after the revolution of the abutting section 14 is formed in an arcuate surface 15 centering around a latch shaft 10.