PURPOSE: To facilitate detection of physical amount along the distributed constant line, by providing a signal at one end of distributed constant line then detecting the waveform obtained at the time point two times that required for said signal to transmit the distributed constant line.

CONSTITUTION: The end of central conductor of distributed constant lne 1 is connected with impedance 4 the other end of which is connected with the sheath conductor of distributed constant line 1 while the start of said sheath conductor is connected together with signal generator 2 with the common. The wave appearing at the joint between the start of distributed line 1 and impedance 3 is provided to waveform detector/processor 5. When a signal is supplied from signal generator 2 to distributed constant line 1 waveform detector/processor 5 detects the reflected waveform then performs predetermined processing to produce a signal analogous with the distribution of physical amount to be measured along the distributed constant line 1.