An electronic control unit is configured in such a way that the groove-shaped concave portion of the second case member includes a first concave portion, in which a groove width at a bottom surface side is narrow, and a second concave portion, in which a groove width at an aperture surface side is wide, and the first concave portion and the second concave portion are linked by an inclined step portion in such a way that a groove width at the step portion is increased in a direction from the bottom surface side to the aperture surface side, and moreover, a tip of the rail-shaped convex portion of the first case member is fitted into the first concave portion at the bottom surface side of the second case member.