To provide a lightning protection circuit which protects against an induced lighting stroke and also has countermeasures against an electromagnetic wave interference.

A lightning protection circuit of an unbalanced transmission system comprises an arrester ARR1 of a three-pole arrester inserted between the lines of a transmission line whose another end is connected with a frame ground, a varistor V1 inserted between the lines of the transmission line, a varistor V2 inserted between a signal ground and the frame ground, and a resistor R1 inserted in series on signal line side with suspect to the transmission line. Also, a B/U CONV1 and U/B CONV2 are respectively the receiving and transmitting portions of a transmission equipment.