An insulation rib is provided above a charging circuit and a plurality of heat radiation ribs are provided on the left side of the charging circuit. A space part is formed by the insulation rib. Little of the heat generated by the charging circuit is transferred to an upper sidewall portion of a case main body portion due to a heat insulation effect of the space part. Further, heat generated by the charging circuit is likely to be transferred to a left sidewall portion of the case main body portion due to a high heat dissipation effect of the heat radiation ribs. In this way, the generation of locally heated areas in the case main body portion can be reduced through balancing of the flow of heat generated by the charging circuit.