To improve the burglarproof performance of a crescent lock by toughening an unlocking work by a malfeasance intruder from an opening section obtained by partially cracking a glass plate.

The crescent lock has a receiving member 12 fitted to a vertical frame 7 for an outdoor sliding door 4 and a crescent member 14 fitted rotatably around an axial center X in the traveling direction of the sliding doors 3 and 4 to the vertical frame 6 for the indoor sliding door 3 superposed to the vertical frame 7 for the outdoor sliding door 4. In the crescent lock, both indoor and outdoor sliding doors 3 and 4 are locked by engaging the crescent member 14 with the receiving member 12 by rotating the member 14. The crescent lock has an operating member 15 mounted to the vertical frame 6 for the indoor sliding door 3 so as to be freely rotated around the axial center Y in the indoor and outdoor direction. The crescent lock further has an interlocking mechanism 37 converting the rotary motion of the operating member 15 into that of the crescent member 14 and an operating knob 16 detachable from the indoor side in the operating member 15.