A radon-thoron discriminative passive dosimeter has been developed that can estimate both radon (222Rn) and thoron (220Rn) concentrations at the same time. Two polycarbonate films are installed as solid-state nuclear track detectors in the dosimeter housing. One film registers alpha tracks originating from predominantly thoron and its progeny together with a small contribution from radon, and the other film registers alpha tracks originating from radon and its progeny together with a negligible contribution from thoron. The lower detection limit is estimated to be 2.9 Bq m−3 for the radon concentration and 9.0 Bq m−3 for the thoron concentration for 2 mo exposure. Preliminary measurements from 1991–1992, in a traditional Japanese wooden house located in Kyoto, indicated that the indoor thoron concentration increases exponentially as the interior mud (or plaster-coated) wall is approached. A soil-based plaster commonly used in Japanese wooden houses to fill walls (or as a surface coating on the walls) is the probable source of the indoor thoron. Since thoron is not measured by the usual radon measurements, and the majority of Japanese houses are made of wood, attention should be paid to indoor thoron and its decay products, which may give a significant fraction of the total natural radiation exposure to the general public.