A lamp with a single base is mounted on a lamp holder by processes of contacting a second mounting face of the base with a first mounting face of the lamp holder and turning the base in a first turning direction with respect to the lamp holder. A plurality of engaging projections is formed on the lamp holder with which a plurality of engaging protrusions of the base is engaged, when the base is turned in a first turning direction. A pair of hooking receivers is formed on a mounting face of the lamp holder so as to be hooked with a hooking protrusion of the base. A pair of lamp holding springs is provided on the lamp holder for applying forces for restricting turning of the base in a second turning direction opposite to the first turning direction under a condition that the second hooking portion of the base is hooked with the first hooking portion. Thus, safety and reliability of mounting of the lamp on the lamp holder can be increased.