PURPOSE: To improve detection efficiency, by controlling the pressure of the sample discharged gas in a container wherein a radiation detector is arranged to a specific high and a low value, and detecting radiation.

CONSTITUTION: When the pressure in the container 7 attains to the 1st pressure PH (at time t1), a controller 22 reads the pressure PH and closes an intake valve 11. The pressure in the container is held at the 1st pressure PH until time t2. An discharging valve 12 is opened at the time t2 and when the pressure in the container 7 drops to the 2nd pressure PL (at time t3), the discharging valve 12 is closed and the pressure in the container 7 is held at the 2nd pressure PL up to time t4. A radiation measuring instrument 23 integrates and stores outputs IH and IL of a radiation detector 8 corresponding to the pressures PH and PL only in a specific time Δt in the holding period (t1Wt2) of the 1st pressure PH and the holding period (t3Wt4) of the 2nd pressure PL. Further, the output I0 of the radiation detector 8 converted into the reference pressure P0 in the container 7 is calculated from an equation.