PURPOSE: To simply detect a failure of a power transmission system by generating the output of a measuring timing with zerocross signal for detecting the zero time points of the instantaneous values of respective transmitting current and receiving current, at a power transmitting station and a power receiving station, and by measuring the transmitting and receiving current at the time points to compare them with each other.

CONSTITUTION: A power transmitting station 10 and a power receiving station 20 are connected to each other with a transmission line 5. From the output of the generator 11 of the power transmitting station 10, a first zerocross is detected 12, and a first timing pulse is generated 14, and by a first measuring circuit 16, the instantaneous value of transmitting current is found. The found instantaneous value is transmitted 19 to a comparator 30 via a first signal- transmitting circuit 18. In the quite same manner as at the power receiving station 20 also, from receiving current, a second zerocross is detected 22, and a second timing pulse is generated 24, and by a second measuring circuit 26, the instantaneous value of the receiving current is found. The found instantaneous value is transmitted 29 to the computor 30 via a second signal-transmitting circuit 28. By the comparator 30, the output 19 and the output 29 are compared with each other, and if there is an abnormal state, alarm is given. As a result, by a simple circuit for requiring no main and subordinate relationship, a failure in a system is detected.