A unique system for monitoring radioactivity in exhaust gas has introduced to the Radon experiment facility at National Institute of Radiological Sciences (NIRS), which has dual detector sets at the air intake and the terminal exhaust duct. The concentration of radon generated in the facility could be evaluated from the difference in the count rate between two detectors. Under ventilation, the radon concentration of both outdoor and exhaust air was almost same, and the equilibrium-equivalent concentration (EEC) was estimated to be nearly 0 Bq m-3. When the air conditioner was halted, radon EEC in exhaust air became slightly lower than that in outdoor. The difference was estimated to be caused by attachment of the radon progeny to the exhaust ducts and sampling ones.