PURPOSE: To specify a faulty section if a fault occurs in a section where the monitoring is not carried out between the devices or in other sections in a path monitoring system which monitors the communication state of a signal path set in a device or between the devices.

CONSTITUTION: A monitoring bit inserting part 1 is provided to insert a correct monitoring bit to a main circuit 6 of an input part at the relevant time point together with a path monitoring part 2 which monitors the monitoring bit via the circuit 6, a path monitoring bit reinserting part 3 which reproduces a monitoring bit similar to that of the part 1, and a selector part 4 which detects the monitoring bit passed through the circuit 6 at a path detecting part 2, transmits the signal of the circuit 6 as it is if a fault is detected, and then selects the monitoring bit of the part 3 and sends it to the following circuit 6 if no fault is detected. Thus the part 4 decides to transmit the original monitoring bit to the following stage as it is or to transmit the monitoring bit inserted to the part 3.