A drain nut attachment structure comprising: a synthetic resin cover (10; 100); and a drain nut (11; 110) made of a metal material and attached to the synthetic resin cover (10; 100), characterized in that: the drain nut (11; 110) comprises: a nut portion having a noncircular outer circumferential surface; a female thread portion formed in the center of the nut portion such that a drain plug is screwed in the female thread portion; a cylindrical portion formed concentrically with the nut portion; and a large-diameter portion formed coaxially between the nut portion and the cylindrical portion and having a cylindrical outer circumferential surface larger in diameter than the cylindrical portion and smaller in diameter than the nut portion; the synthetic resin cover (10; 100) comprises: a nut engagement portion formed in a noncircular shape to be open to an outer surface of the synthetic resin cover (10; 100) such that the nut portion is engaged in the nut engagement portion; a stepped portion formed to be open to a bottom surface of the nut engagement portion and having a cylindrical inner circumferential surface; a through hole formed to be open at one end thereof to a bottom surface of the stepped portion; and a cylindrical engagement portion formed on the inner surface of the synthetic resin cover (10; 100) and having a plurality of piece parts disposed circumferentially at predetermined intervals in an annular arrangement around the through hole; an O-ring is arranged to establish a radial seal between the inner circumferential surface of the stepped portion and the outer circumferential surface of the large-diameter portion; and the drain nut (11; 110) further comprises a collar portion formed on a front end of the cylindrical portion and protruding radially outwardly from the cylindrical portion so as to be retained by front ends of the piece parts. a synthetic resin cover (10; 100); and a drain nut (11; 110) made of a metal material and attached to the synthetic resin cover (10; 100), characterized in that: the drain nut (11; 110) comprises: a nut portion having a noncircular outer circumferential surface; a female thread portion formed in the center of the nut portion such that a drain plug is screwed in the female thread portion; a cylindrical portion formed concentrically with the nut portion; and a large-diameter portion formed coaxially between the nut portion and the cylindrical portion and having a cylindrical outer circumferential surface larger in diameter than the cylindrical portion and smaller in diameter than the nut portion; a nut portion having a noncircular outer circumferential surface; a female thread portion formed in the center of the nut portion such that a drain plug is screwed in the female thread portion; a cylindrical portion formed concentrically with the nut portion; and a large-diameter portion formed coaxially between the nut portion and the cylindrical portion and having a cylindrical outer circumferential surface larger in diameter than the cylindrical portion and smaller in diameter than the nut portion; the synthetic resin cover (10; 100) comprises: a nut engagement portion formed in a noncircular shape to be open to an outer surface of the synthetic resin cover (10; 100) such that the nut portion is engaged in the nut engagement portion; a stepped portion formed to be open to a bottom surface of the nut engagement portion and having a cylindrical inner circumferential surface; a through hole formed to be open at one end thereof to a bottom surface of the stepped portion; and a cylindrical engagement portion formed on the inner surface of the synthetic resin cover (10; 100) and having a plurality of piece parts disposed circumferentially at predetermined intervals in an annular arrangement around the through hole; a nut engagement portion formed in a noncircular shape to be open to an outer surface of the synthetic resin cover (10; 100) such that the nut portion is engaged in the nut engagement portion; a stepped portion formed to be open to a bottom surface of the nut engagement portion and having a cylindrical inner circumferential surface; a through hole formed to be open at one end thereof to a bottom surface of the stepped portion; and a cylindrical engagement portion formed on the inner surface of the synthetic resin cover (10; 100) and having a plurality of piece parts disposed circumferentially at predetermined intervals in an annular arrangement around the through hole; an O-ring is arranged to establish a radial seal between the inner circumferential surface of the stepped portion and the outer circumferential surface of the large-diameter portion; and the drain nut (11; 110) further comprises a collar portion formed on a front end of the cylindrical portion and protruding radially outwardly from the cylindrical portion so as to be retained by front ends of the piece parts.