To provide a bolt fastening structure and a seat load measurement device capable of relaxing impact of contact of a screw part of a bolt and a bolt hole.

The seat load measurement device is provided with a plurality of load cell units 2 provided at a lower part of the seat 1; and brackets 3, 3 connected to both ends of the respective load cell units 2. It is fixed to a vehicle body side by fastening the bolt to the bolt holes 3a, 3b bored on the respective brackets 3, 3. The bolt hole 3b is bored in the larger form than an outer diameter of the screw part 21 of the bolt, and an opening 23 is bored so as to form a band part 22 on a periphery of the bolt hole 3b.