PURPOSE: To relieve the load of a checker and to increase work efficiency and reliability of a terminal block by installing a circuit which detects the separation between two electrodes of an detecting element when a screw is loosened and a circuit which sounds an alarm based on the detected signal.

CONSTITUTION: A detecting element 16 comprising two electrodes 15a, 15b placed on both sides of an insulating frame 14 between insulating plates 13a, 13b is installed. A wire 7 is interposed between a plate washer 4 and a spring washer 3 and fixed with a screw 2. If the screw 2 in one terminal of many terminals is loosened, the electrode 15a of the detecting element in the loosened terminal is reparated from the electrode 15b and they are electrically insulated. A resies circuit of all detecting elements 16 in a terminal block is discontinued, and a relay 11b of a detecting circuit 11 becomes free from excitation. A contact 11c is thereby closed and an alarm circuit is connected. The load of a checker is relieved and the reliability of the terminal block is increased.