To fully close a sliding door and maintain a locking condition as it is by such a simple operation as the closing of the sliding door in the case of the slight positional displacement of the sliding door.

This device is composed of a sickle catcher 3 installed within a stile/rail 2 of one sliding door 1 and the other sliding door 6 provided with a sickle piece 4 free to be engaged with/disengaged from the sickle catcher 3 within a stile/rail 5 facing the stile/rail 2. The sickle catcher 3 is provided with a ratchet tooth-like engaging part 12, wherein the position to engage the sickle piece 4 is changed when the right and left sliding doors 1 and 6, respectively, are moved in a closing direction, and journalled on a fixed shaft 13 mounted horizontally and supported rotatably on the stile/rail 2 in such a manner as to be able to rotate against the spring force of a spring member 10. The sickle piece 4 is rotatably supported within the stile/rail 5 through the intermediary of a fixed supporting shaft 30 for the purpose of being rotated by the operation of an operating member 23 installed in such a manner as to slide freely within the stile/rail 5 and having its end part 4a engaged selectively with the engaging part 12.