According to one embodiment, a protective relay performs the following steps. A/D converter 11 acquires an electric current of a power transmission line 4 via a CT 5. A computing part 12 performs various protection computations such as current differential computation using a current data measured at the end of itself and a current data measured at the opposite end. A transmitting and receiving circuit 13 transmits the current data of itself to the other protective relay 1 at the opposite end via a transmission path 3. A received level detection part 14 detects the received level of the transmission data that has been transmitted from the opposite end and received by the transmitting and receiving circuit 13. A transmission level control part 15, based on the difference between the received level that has been detected within a definite time t1 after the establishment of data transmission to the other protective relay 1 and a predetermined received level, performs transmission level reduction control after the passage of the definite time t1.