An electronic control unit 31 for mounting on a vehicle include a control portion 32 and a power portion 33 which are separate from each other, and are combined together. A control board 40 is mounted on a control connector-incorporating casing 34, having control connectors 35 and 36 formed integrally thereon, thereby forming the control portion 32. Connection terminals 41 on the control board 40 are soldered to connector connection terminals 37 and 38, embedded in the casing 34, and connection terminals 42 on the control board 40 are soldered to relay connection terminals 39 embedded in the casing 34. Power parts 57 and 58 are mounted on a power connector-incorporating casing 54, having power connectors 55 and 56 formed integrally thereon, and further a heatsink is mounted on the casing 54, thereby forming the power portion 33. An intermediate layer 51 can be provided between the control portion 32 and the power portion 33. Parts can be mounted on the intermediate layer 51.