To provide a lock device which can improve fireproof performance by closing a clearance formed in a lock case in the case of fire.

A lock device 13 is provided with a lock case 19 which is provided in the door end of a door 15, a cylinder lock 21 which is inserted through a door hole 29 formed in the door 15 and mounted to a mounting hole 27 formed in a lock case side plate 25, and a thermal expansion refractory material which is provided between a cylinder lock outer peripheral surface 67 and the lock case side plate 25 and expanded to close a clearance 69 between the cylinder lock outer peripheral surface 67 and the mounting hole 27 when heated. The thermal expansion refractory material is formed as a belt-like winding refractory material 65, which can be wound around the cylinder lock outer peripheral surface 67 in the form of a ring so that the longitudinal direction becomes the circumferential direction of the cylinder lock outer peripheral surface 67. The thermal expansion refractory material is formed as a circular plate-like sticking refractory material, which may be stuck to the peripheral edge of the mounting hole 27 in the outer surface 75 of the lock case side plate 25.