To provide a fastening device which is not easily disengaged when the device is fastened to a bolt, and prevents slackness caused by vibration or an external force for a long period of time.

The fastening device includes a coil spring 13 comprising a small diameter coil 13a housed inside a spring housing 11, a large diameter coil 13b mounted to the outside of the spring housing, and a coil connecting portion 13c connecting the small diameter coil with the large diameter coil. The spring housing has a guide notch 11a formed from an opening side in an axial direction. The coil connecting portion is inserted into the guide notch, making the end 13b1 of the large diameter coil as a free end. The small diameter coil can be screwed into the male screw groove of the bolt while the end 13a1 of the small diameter coil is made to be a free end. A press-moving portion 11b1 is formed on the inner wall of the spring housing, the press-moving portion pressing and moving the end of the small diameter coil by rotation after canceling the inhibition of rotation. The engagement of the large diameter coil with the spring housing is released by a rotational force larger than the force of inhibiting rotation, the end of the small diameter coil is press-moved by the press-moving portion, and a lock nut is detached from the bolt.