To surely detect loosening of a terminal screw by preventing floating of an insulation spacer interposed between a terminal seat and a nut in a terminal structure for detecting loosening of the terminal screw on the basis of a potential difference between the terminal seat and the nut.

A terminal groove 4 is formed in a housing 2 of a circuit breaker, a nut 16 is fixed onto a bottom of the terminal groove 4, and a terminal seat 14 is placed on the nut 16 via an insulation spacer 17. A terminal screw 12 is passed through a hole 15 of the terminal seat 14 and threaded to the nut 16, and a connection conductor 8 of an electric wire 7 is fastened to the terminal seat 14 by the terminal screw 12. A potential difference between the terminal seat 14 and the nut 16 is detected and when the potential difference exceeds a specific value, loosening of the terminal screw 12 is determined. In a height equal to an upper end face of the nut 16, a locking hole 20 is formed on an inner surface of a sidewall 3. The insulation spacer 17 includes a pair of overhanging parts 21, and locking claws 22 are projected in the overhanging parts 21. The locking claws 22 are fitted into the locking hole 20, and the insulation spacer 17 is held at a position covering the upper end face of the nut 16.