To provide a fastening structure, preventing deterioration of flatness of a flange surface when serration of a bolt is pressed in the flange.

In this fastening structure, a bolt 5 having serration 7 formed on an axial portion of the outer peripheral surface 6 is pressed in a bolt hole 3 formed in the flange part 2 of a fitting member 1. The bolt hole 3 of the flange part 2 is composed of a first enlarged part enlarged to be tapered toward the bearing surface of the flange part 2; a central small-diameter part; and a second enlarged part enlarged to be tapered toward the above disk brake side. The serration 7 of the bolt 5 is engaged with the central small-diameter part of the bolt hole 3 of the flange part 2. The serration 7 of the bolt 5 is entirely engaged with the central small-diameter part of the bolt hole 3 of the flange part 2.