PURPOSE: To execute the monitor of a signal processing circuit with small scale hardware configuration by inserting a monitor pattern to plural input signals and allowing a selection circuit to select and output all monitor patterns in circulation.

CONSTITUTION: Monitor pattern insertion circuits 11-1n insert a monitor pattern to a prescribed position of input signals IN#1-IN#n inputted from plural lines. A selection circuit 20 is controlled by timing control of a timing signal generating circuit 40. One signal is selected as a main signal and then outputted, and all the monitor patterns are circulated and selectively outputted to a signal processing circuit 100. Then the circuit operation from the input to the circuit 20 till the output of the circuit 100 is monitored by checking the monitor pattern outputted to the circuit 100, and when a fault is detected, an alarm signal ALM is outputted. Through the constitution above, the circuit monitor system is realized, in which the signal processing circuit is monitored with a small scale hardware.