In an electronic control apparatus, a connector mounted on a circuit board accommodated in a casing body constructed with top and bottom casings is partially exposed outside the casing body to be mated with an external connector. A sealant is interposed between the top and bottom casings, between the top casing and the connector, and between the bottom casing and the connector. A sealing part of at least one of the top and bottom casings has a substantially rectangular ring shape with rounded four corners. A sealing part of the connector faces two adjacent corners of the rectangular sealing part and one side of the rectangular sealing part directly connecting the two adjacent corners.