To provide a method and device for detecting the existence and position of a foreign matter staying within a liquid transporting pipeline by means of a pressure wave produced in the pipeline and its reflection wave.

A zone in the liquid transporting pipeline 1 from one position to the position where the foreign matter stays is pressurized by a booster pump 9. A negative pressure wave is produced by using a pressure producing nozzle 4 and a pressure-wave producing valve 5. A liquid pressure in the pipe is measured by a pressure transmitter 6. By using thus measured values, a data collecting/processing unit 8 measures a lapse of time between when the negative pressure wave is detected and when its reflection wave is detected. Based on thus measured lapse of time and the propagation velocity of the pressure wave and reflection wave, the existence and position are detected of the foreign matter 2 staying within the pipeline 1.