JPS627514B

PURPOSE:To facilitate the measurement of the concentration of a radioactive gas, by measuring the radioactivity of the radioactive gas collected by a filter cartridge at a radioactivity measuring unit, by measuring the flow rate thereof within a piping at a flow-rate measuring unit, and by measuring thereby the concentration of the radioactive gas. CONSTITUTION:Iodine is collected by adsorbing to a filter cartridge 5 at a collecting unit 4. The level of the radioactivity of the iodine is monitored by a level monitor 9. While the flow of sample air is stopped, purge air or the like is supplied to a piping 1 via a purge gas supply pipe 17 and made to flow through the filter cartridge 5. Rare gas or the like, other than the iodine, adhering to the collecting material of the filter cartridge 5 is thereby blown away. Then, the cartridge 5 is transferred to a measuring unit 21 and the radioactivity is measured by a high level monitor 10 or a low level monitor 11. The concentration of the iodine in the sample air is computed in an operator 14, based on the outputs of the level monitors 9, 10 and 11 and the output of a flow-rate measuring unit 6.