PURPOSE: To shorten the cleaning operation time while reducing the quantity of a cleaning fluid by cleaning a piping with intermittently repeated passage of the cleaning fluid there-through.

CONSTITUTION: An offgas piping 5 form a turbine 4 operated by a steam from a nuclear reactor 3 is connected to an exhaust cylinder 7 with a conduit through an offgas treating device 6. When the measurement ends on activity of the offgas, inlet valves 51 and 52 of a sampling piping 8 are closed while top valves 84 and 85 of a cleaning gas piping 12 are operated. A cleaning gas selector valve 18 is opened by operating a valve opening/closing controller 18 to allow the cleaning gas to flow from a cleaning gas tank 16 and then closed after a fixed flowing time elapses. Then, after the passage of a flow stoppage time, the controller 18 is actuated to perform the same cleaning gas flowing operation. This procedure is repected. After finishing the cleaning, the concentration of radioactive substance in the cleaning gas at the outlet of the sampling piping 8 is measured and determined with a measuring device 9.