A circuit board is hermetically-sealed and housed in a steel case that is composed of a metal base and a metal cover, the cover disposed opposite to a first board surface has a tall flat portion disposed opposite to a connector housing and a short flat portion disposed opposite to a heating component, and the heat generated from the heating component is directly transferred to a heat transfer base portion of the base via a heat transfer mechanism and a heat transfer filling material. Surface finishing in which the coefficients of heat radiation are mutually 0.7 to 1.0 is applied to the surface of the heating component and the opposite inner surface of the cover, and radiation and heat transfer are efficiently performed to the short flat portion of the cover.