PURPOSE: To increase the rotation preventive force of a key cylinder without increasing the thickness of a mounting panel.

CONSTITUTION: A face 19a corresponding to a first projection 19 in the rotation direction R of the first projection 19 and an edge 18a of a first notch section 18 corresponding to the face 19a make nearly same acute angle θ2 to the tangential line L of a mounting hole 17 in the intersection S1 of the edge 18a and the mounting hole 17 in the rotation direction R respectively. Therefore, even if the face 19a is engaged with the edge 18a at strong force corresponding to the rotation force of a key cylinder 2, the rotation force is concentrated mainly to a section S2 corresponding to an edge, which has high strength. Therefore, the edge 18a will not deform.