  A device for measuring radioactive impurities in gas is developed, consisting
of an electrostatic collection type radon detector and an extremely low
radioactivity proportional counter. This device measures the radioactive
impurities released by materials into the gas and a filtering test for radon
removal from gas. The performance of the newly developed proportional counter
was evaluated by measuring radon rate. The calibration factor of $^{222}$Rn as
27.4$\pm$0.2~count/day/mBq and the background rate of $^{222}$Rn as
0.84$\pm$0.13~mBq were estimated.
