To reduce a consumption of a long filter paper while securing prescribed measuring precision for a concentration of a radioactive nuclide.

A first suction part 31 and a second suction part 32 are formed in a suction head 3a to partition a width direction of the long filter paper 1 into two dust capturing areas, a first solenoid valve 33 is connected to the suction part 31, a second solenoid valve 34 is connected to the suction part 32, and pipings from the both solenoid valves are integrated into one in a flow measuring side. The solenoid valves 33, 34 are switched to operate the suction parts 31, 32 separately, and the two dust capturing areas corresponding to both the suction parts 31, 32 of the long filter paper 1 are used independently to make an effective filter paper length double. A conveying speed of the filter paper 1 is slowed down to cope with a negative suction pressure value of a suction pump and a radiation detection signal from a radiation detector.