A casing for an electronic control device, by which a cover and a casing body are reliably sealed up while preventing the cover from warping or from being distorted, although the cover is made of a thin sheet material. The electronic control device casing comprises: a casing body having an opening in its upper face portion for casing a printed circuit board; a cover made of a sheet material for closing the opening of the casing body; a groove portion formed in the open edge portion of the casing body; a liquid sealing material fitted in the groove portion for sealing the open edge portion of the casing body and the flange portion of the cover; and a ridge portion having a generally U-shaped section and formed on the flange portion of the cover for being fitted in the groove portion of the casing body.