A key cylinder installation structure includes a key cylinder including a flange that is fixed to a trunk of a vehicle, the flange including a mounting hole through which a mounting bolt is inserted into a reference hole and a secondary reference hole, respectively, formed in the trunk, the key cylinder being operable to lock and unlock the trunk, and a pad that is attached to the flange of the key cylinder and includes a hole through which the mounting bolt is inserted and a projection projecting toward a center of the key cylinder in the hole of the pad. When the mounting bolt is inserted into the mounting hole, the projection of the pad lies between the mounting bolt and the mounting hole.