To provide a removal prevention device which surely prevents a bolt from being removed from a fastened object.

A coil spring 2 to be screwed in a screw of a bolt is accommodated in a hex head nut 1. One end of the coil spring 2 is locked to the recess 1b provided to one end surface of the hex head nut 1. The inner peripheral surface 1a of the hex head nut 1 is made flat. In this configuration, when relatively rotating the hex head nut 1 in the reverse direction to the screw of the bolt, one end 2a of the coil spring 2 is transformed to rupture, which leads to a freewheeling condition of the hex head nut 1 with respect to the screw of the bolt and the coil spring 2.