To surely and stably acquire a door closing signal even if the size of a gap is changed correspondingly to a rebound at the time of abruptly closing a sliding door while reducing the number of parts.

In an ON/OFF mechanism for a switch of the sliding door lock wherein the door closing signal of the switch is picked up by a controller at the time of a door closure, i.e., a collision of the leading end of the door against a sash frame, the mechanism comprises a switch 60 disposed within a lock case 10, a trigger 31 provided in the case 10 slidably in the horizontal direction and a swing arm 51 pivoted between the trigger and the switch. The swing arm 51 has a collision part 52 which is provided on one end thereof and collides against an impact face 46 formed on the trigger 31. When the impact face 46 of the trigger 31 pushed by a receiving member Y to recede collides against the collision part 52, the swing arm 51 swings about a pivot 26 to operate the switch by the other end thereof located on the side opposite to the collision part 52. On the other hand, the collision part 52 rides on the top face 36 of the trigger 31 and is kept in the riding state.