An ECU has an enclosure which is contained in the casing of an automatic transmission, which is formed of a material having low heat conductivity, and which provides liquid-tight containment of a substrate. A heat sink is attached to the substrate so as to dissipate heat from an electronic circuit on the substrate. The heat sink is attached in contact with an inner wall surface of the enclosure, so that heat produced by the electronic circuit is dissipated via the heat sink and the enclosure.