Abstract: In a ventilation mechanism of an electronic control device, a ventilation path is formed by a through hole and a window formed in a side surface of a projection projecting outward of a case, and entry of water into the ventilation path is prevented by a wall. A water-repellent filter is attached to an inner wall of the case, with a gap between the water-repellent filter and a bottom surface of the projection. The projection and the wall are integrated with the case. This achieves reduction in the number of components of the ventilation mechanism and improvement in machinability while ensuring a waterproof function.