To provide a fastened member capable of keeping a fastened state with a base material by a tamper-proof set screw.

A second cover 15 fastened with a housing 12 by a tamper-proof set screw 24 has a contact surface 17 to which the tamper-proof set screw 24 is contacted and a projection wall 171 near the contact surface 17. A side face 172 of the projection wall 171 is formed to become an arcuate shape having a central angle θ of 180° around an axis φ of the tamper-proof set screw 24 after the second cover 15 is fastened to the housing 12 and at the same time a distance L1 between a point 174 nearest to the outer-most point 245 of a head part 241 of the tamper-proof set screw 24 and the outer-most point 245 becomes 1.3 mm. With the foregoing, it does not become possible to insert a tool such as a radio pinch between the head part 241 of the tamper-proof set screw 24 and the side face 172 of the projection wall 171, resulting in that a fastened state between the housing 12 and the second cover 15 can be maintained.