To provide an internal thread structure for fastening a resin molding thinned in wall thickness without causing a crack of the resin molding in fastening the resin molding with a tapping screw.

The internal thread structure 10 for fastening the resin molding fastens the resin molding 2b to a mating member 2a with the tapping screw 9. The resin molding 2b comprises a prepared hole forming outer enclosure part 21 having a prepared hole 22 for guiding the tapping screw 9 to screw the tapping screw 9 to the resin molding 2b, and the outline 23f of a sidewall 23 partitioning the prepared hole 22 of the prepared hole forming outer enclosure part 21 is formed in a polygon, and the prepared hole forming outer enclosure part 21 is provided with support ribs 24 formed extending outward from a polygonal face part 23h.