A fastening structure capable of preventing deterioration in the flatness of a surface of a flange portion (2) when a serration (7) of a bolt (5) is press-fit into the flange portion (2). In this fastening structure, one portion in an axial direction of an outer peripheral surface (6) of the bolt (5) is provided with the serration (7). The bolt (5) is press-fit into a bolthole (3) formed through the flange portion (2) of a mounting member (1). The serration (7) is located apart from respective end surfaces (2A), 2B) of the flange portion (2) by first and second distances (D, C), and the first and second distances (D, C) each exceed 13% of the thickness (A) of the flange portion (2). With this arrangement, the serration (7) presses a bolthole inner peripheral surface (13) only in the region located far from both end surfaces (2A and 2B) of the flange portion (2).