A circuit board mounting electronic components including a heat generating element is accommodated in a casing. A heat dissipation member is disposed between the heat generating element and an inner surface of the casing. The heat dissipation member is thermally bonded to the heat generating element and to the casing. The heat dissipation member is a polymeric material having fluidity. A film for preventing a shifting of the heat dissipation member is disposed between the heat dissipation member and the casing. And, the film is chemically or electrically bonded to the heat dissipation member and to the casing.