To provide a fastening structure capable of preventing degradation of flatness of a flange face when a serration of a bolt is pressed into a flange.

In this fastening structure, the bolt 5 having the serration 7 in an axial part of the outer peripheral surface 6 is pressed into a bolt hole 3 formed in the flange section 2 of an attaching member 1. The serration 7 is separated from both end faces 2A and 2B of the flange section 2 by first and second distances D and C. The first and second distances D and C exceed 13% of the thickness A of the flange section 2. Therefore, the serration 7 presses the inner surface 13 of the bolt hole only in a region far from both end faces 2A and 2B of the flange section 2.