Background: Separate radon and thoron measurements in soil are very important in assessment of internal exposure due to inhalation of such radioactive gases. Materials and Methods: In this study, a low cost, small size, passive diffusion chamber has been developed for simple measurement of Radon-222 and Radon-220(Thoron) gases separately in soil. The diffusion chamber consists of two films and two fiber glass filters. Lexan polycarbonate films were used as Solid State Nuclear Track Detectors (SSNTDs) and optimized film to filter distance was obtained. Results: Calibration factors for the designed diffusion chamber were measured using flow through method which was 16.85[track.cm^-2(kBq. m-3d)^-1] and 17.25 [track.cm^-2(kBq.m^-3d)^-1] for radon for the lower and upper Lexan films, respectively and 1.76 [track.cm^-2(kBq.m^-3 d)^-1] for thoron. Conclusion: The designed chamber is an economic, applicable and efficient detector for measurement of radon and thoron separately in soil.