To provide a lock device for an outdoor-side sliding door using a sliding door lock, which has the high flexibility to change the thickness of the door.

This lock device is housed in the outdoor-side sliding door 3 in a state in which a slider 2 and a pair of control levers 7 and 7' are arranged in a lock case 8. The slider 2 is vertically driven by operating a cylinder lock 1. Either of the pair of control levers 7 and 7' is selected and rotatively driven depending on the direction of driving the slider 2, and gets into an indoor-side sliding door 4 from an initial position in the sliding door 3, so that a sickle-shaped locking member 5 arranged in the sliding door 4 can protrude from/recede into the engage-in hole 6 of the sliding door 3. The cylinder lock 1 is fixed to a decorative seat 22 which is fixed to an outdoor-side wall surface in the state of making a key inserting opening exposed to the surface of the sliding door 3. The slider 2 is formed in such a shape that rectangular openings 2b, in which teeth of a pinion 23 for being rotated by a rotor 1b of the cylinder lock 1 are meshed with one another, are opened with a predetermined pitch, and that a square core fixed to the end of the rotor 1b of the cylinder lock 1 is fitted into a square hole axially penetrating the center of the pinion 23.