Due to its short half-life, radioactive thoron gas has been assumed to have negligible health hazards on humans compared to well-known radon gas. But, one of the decay products with a long half-life can make it to be transported to a long distance and to cause a severe internal dose through respiration. Since most commercial radon detectors can not discriminate thoron signals from radon signals, it is very common to overestimate radon doses which in turn result in a biased estimation on lung cancer risk in epidemiological studies. Also though some methods had been suggested to measure thoron and radon separately, they could not be used in a continuous mode because they used CR-39 or LR-115 requiring an etching process later. In this study, a method of two ion chambers and an air-flow delay was suggested to measure radon and thoron separately from the free air. It was observed that the activity concentration of thoron in the gas decreases quickly and exponentially within a delay time of a few minutes caused by a long pipe between two chambers while radon concentration remains almost the same. Therefore from two ion chambers apart in time, it was demonstrated that thoron and radon could be measured separately and simultaneously. I also developed a collimated alpha source and with this source and an SBD, one could convert the ion chamber reading to count rate in cps. The proposed system can be a practical solution to detect radon and thoron separately and in a continuous mode which has not been demonstrated yet.