To surely detect leakage of liquid, gas, etc., from piping reliably at any location even when leakage is extremely small without performing an auditory inspection or a visual inspection.

An orifice 5 in which a differential pressure detecting unit is embedded is inserted into a piping. Then, as a fluid is made to flow on the upstream side from the piping in a state that air is injected in a tube 6 provided on the periphery of the orifice 5 and while moving the orifice 5 to the downstream side, differential pressure in the piping is detected by the differential pressure detecting unit provided in the orifice 5, and the result of detection are stored together with time information.