To enable locking and unlocking to be performed from an indoor or outdoor side, or from either of the indoor and outdoor sides, according to an installation place or needs.

A protruding/receding member 3, which moves to the side of a door end stile by abutting on a frame body along with the closing motion of the sliding door 2, is provided in protrusively urged state on the door end stile 2A of the sliding door 2 which is openably/closably provided in the frame body 1. This lock is equipped with a first interlocking mechanism 6 for locking a latch 4, provided in the door end stile, to a locking part 5, provided in the frame body 1, by protruding the latch 4 along with the receding motion of the protruding/receding member 3. The lock is equipped with an operation part 8 for operating a locking member 7, which inhibits the operation of the latch 4 to an unlocking side by abutment, from an indoor side. The lock is equipped with at least either of a second interlocking mechanism 11 for operating the locking member 7 while being interlocked with an opening assisting mechanism 9 for reducing and assisting the initial motion of the sliding door 2 to the opening side, and a third interlocking mechanism 13 for interlocking the locking member 7 with a cylinder lock 12 for enabling the locking member 7 to be operated from the outdoor side of the sliding door 2.