An electronic control device enabling heat generated by a power module to be favorably radiated even in a high-temperature condition in an engine compartment. The unit includes a casing accommodating a control circuit substrate; a die-cast mounting plate forming a control circuit substrate support portion, a frame-like fitting member and an external mounting portion integrally together; and a metal substrate on which a power module is adhered. When the mounting plate is mounted on the wall of a transmission and the metal substrate is fitted in the opening in the bottom surface of the frame-like fitting member, a mounting surface in nearly the same plane is formed by the lower surface of the mounting plate and by the lower surface of the metal substrate permitting heat from the power module to be radiated to the wall.