PURPOSE: To raise reliability of a device whose logical circuit is a part of its constitution, and to execute easily its maintenance, by detecing automatically a fault of the logical circuit by a time counting circuit, and informing the occurrence of the fault to the outside by an indicating circuit.

CONSTITUTION: Specified signals (a), (b) such as a data signal, a control signal, etc. are exchanged to each other among function blocks 1W3 constituted of an LSI. These specified signals (a), (b) are inputted to a decoding circuit 4. The signal (b) selects in advance a signal for generating a positive pulse once at Ta time, and the signal (a) selects in advance a signal which always takes a positive value at the point of time when a positive pulse is generated. The circuit 4 takes AND of the positive values of the signal (a) and (b), and outputs a reset or start signal (c) once at Ta time. When one of the blocks 1W3 generates a fault, and the signal (a) does not take a positive value after tx time, no signal (c) is outputted. Accordingly, a circuit 5 continues time counting, reaches Tb time, outputs a time-out signal (d) to an indicating circuit 6, and raises an alarm.