PURPOSE: To enable early detection of leakage by obtaining an actually measured frequency characteristic pertaining to a wave of a fluid based on a wave parameter of the fluid to determine a difference from a reference frequency characteristic at a band where an impedance mismatched wave appears.

CONSTITUTION: A dispersion type controller 18 reads a pressure value of a fluid passing through a pipe 3 by way of a pressure transmitter 7 to delete the oldest data in a time-series data file and the newest data is stored. The data undergoes a frequency conversion by a Fourier transform to obtain a frequency characteristic of a fluid pressure. Then, a power characteristic at a specific high frequency band where a mismatched waveform of impedance appears is compared with a reference frequency characteristic preset to determine the resulting difference (when a shape of the pipe 3 changes, a difference in the power characteristic is generated with a drop therein at a specified high frequency band). When the difference exceeds a threshold, abnormality is judged to occur and a leak detection alarm is generated in a valve station 16 while a report signal is sent to a central monitoring center 20. The updating of the reference frequency characteristic is performed at an interval of a fixed time (several minute).