An obstacle detecting system having leakage coaxial cables laid on at both the sides of an obstacle monitoring district and an obstacle detecting device for emitting electrical waves from one of the leakage coaxial cables laid on at both the sides to the other leakage coaxial cable and detecting an obstacle in the obstacle monitoring district on the basis of the electrical waves incident to the other leakage coaxial cable is further equipped with an electrical wave transmitting unit for transmitting the electrical waves to the one leakage coaxial cable in the obstacle detecting device, an electrical wave receiving unit for receiving the electrical waves incident to the other leakage coaxial cable, and a connecting unit for connecting the electrical wave transmitting unit and the electrical wave receiving unit.