Disclosed is a structure for attachment of a drain nut to a synthetic resin cover. The cover has a nut engagement portion formed in an outer surface of the cover, a cylindrical engagement portion formed with a plurality of piece parts on an inner surface of the cover, and a stepped portion formed between the nut and cylindrical engagement portions. The drain nut has a nut portion engaged in the nut engagement portion, a female thread portion formed in the nut portion for screw-in of a drain plug, a cylindrical portion formed concentrically with the nut portion and engaged in the cylindrical engagement portion, a large-diameter portion formed between the nut and cylindrical portions and engaged in the stepped portion, with an O-ring held between the large-diameter portion and stepped portion, and a collar portion formed around the cylindrical portion and retained by front ends of the piece parts.