To discriminate an object occupying an intra-pipe space of a pipe line inside which a cable housing pipe is disposed.

A discrimination method is for discriminating an object occupying an intra-pipe space of a pipe line Z by using a laser probe 50. While moving the laser probe 50 along a pipe axis of a cable housing pipe 30 of the pipe line Z, electromagnetic wave pulses are transmitted from a transmission antenna 51, reflected waves of the electromagnetic wave pulses transmitted from the transmission antenna 51 and reflected at a support member 20 are received by a reception antenna 53, a position and arrival time difference curve U for which a position D inside the cable housing pipe of the laser probe 50 and an arrival time difference T from the transmission of the electromagnetic pulses to the reception of the reflected waves are two axes is prepared, and the object occupying the intra-pipe space of the pipe line Z is discriminated on the basis of a feature of the prepared position and arrival time difference curve U.