PURPOSE: To operate a reactor device using a measuring instrument and a device in stock even at the time of obstruction by reducing radiation concentration of gas in a dilusion device in front of a measuring area.

CONSTITUTION: A gas sample is taken out from a sampling opening 6 disposed in a refining gas guide tube 4 (pressure release tube), and guided to a taking-out tube 7. The gas sample is heated over the entire length of the tube 7 with a heater 24. Partial flow from the tube 7 is supplied to a dilusion device 8. The multi-stage constituted device 8 is worked by using the constant volume flow of compressed air including no particles. The first two stages out of the dilusion stages are heated. Dilusion of about 1:104 is performed in the device 8. A measuring guide tube 13 is connected to the final stage, and guided to a measuring area 14. The combination 15 of aerosol and iodine parallel filters for intermittently measuring is provided in the area 14 once a week normally, and at the time of obstruction, measurement is performed every four hours. Therefore, the parallel filters are extracted, and characteristic nuclear species are estimated with a spectrometer in a partition room.