PURPOSE:To enable a cutting position for a repair to be accurately determined by correctly transmitting a ground reference position to an inspection pig and the like moving in a pipeline and storing the correct position of a pitting corrosion in the pig. CONSTITUTION:When an inspection pig moves in a pipeline, a magnetic sensor 40 in the pig detects the magnetism of a magnetic marker on the outer surface of a pipe. While the output of the sensor 40 is amplified (51) and transmitted to an LPF52 and an HPF53, a magnetic mark detection waveform is obtained from which noises are removed by an interaction between the output of an F/V converter that generates a voltage proportional to the moving speed of the pig and filters 52 and 53. An output thus obtained is inputted to a gate circuit 55 controlled by the data set to a voltage comparing circuit 54 and the output of the circuit 55 is inputted to a cross detector 56 to generate a pulse indicative of passing a reference position. Thus, the correct position of a pitting is stored and a correct cutting position for a repair can be determined.
