To provide a lock device which keeps a locked state by preventing rotation of a rotor even if impact force is applied to the rotor in an axial direction.

In a cylindrical rotor (2) into which tumblers (4a, 4b) are inserted, there is formed a protrusion (24) projecting from the outer peripheral surface thereof. In a rotor case (3) which rotatably stores the rotor, a rotation groove (34a) allowing the protrusion to rotate and a lock groove (34b) communicating with the rotation groove to restrict the rotation of the protrusion are formed in a rotor slide hole (33) inside the rotor case. A gate key (6) is undetachably inserted into the rotor case externally so as to cut out the communication between the rotation groove and the lock groove. Thereby, the protrusion of the rotor is held between a gate bar part (61) of the gate key and the lock groove and thus prevented from coming out even if impact force is applied to the rotor.