To increase mounting strength against load applying to the circumferential direction of a key cylinder for a door lock to prevent the door lock from being released by a dishonest act.

A mounting hole 14 having the same outside diameter as that of a case 7 of a key cylinder 3 is formed in a door outer panel 2a of a side door 2 mounting the key cylinder 3 thereto. A cylinder shaft 8 is projected to the rear side of the key cylinder 3 in the front end of the key cylinder 3, and an interlocking shaft 17 is projected to the rear side of the key cylinder 3 in the lower side thereof. A reinforcing mounting plate 5 is placed to the door outer panel 2a at a specific interval in the inside of the side door 2. Circular bearing holes 18 and 19 fitted to the cylinder shaft 8 and the interlocking shaft 17 of the key cylinder 3 are formed in the plate-like mounting plate 5.