A passive dosemeter has been developed which can measure radon and thoron concentrations separately. The dosemeter has a structure of two hemispheric chambers, one 120 mm_ and the other 75 mm_, being combined into a saucer-like shape. Environmental air diffuses into the first chamber of 120 mm_ from the inlets covered with glass fibre filters, from which it passes into the second chamber of 75 mm_. Each chamber contains a disc of polycarbonate film of 50 mm diameter with a 300 µm thickness as an a track detector. A detector film in the first chamber of 120 mm_, is subjected to chemical etching (8N KOH(C2H5OH 20% vol.) at 30 oC) for 3 h and electrochemical etching (AC800V, 2kHz) for 3 h in order to develop a tracks with high incident energy (2.5-4.0 MeV) dominantly originating from thoron and its daughters. Another detector film in the second hemispheric chamber of 75 mm_, is subjected to chemical etching for 30 min. and electrochemical etching for 3 h to develop a tracks with low incident energy (0.9-2.7 MeV) dominantly originating from radon and its progeny. In addition, a pinhole air inlet between the 120 mm_ and 75 mm_ hemispheric chambers causes the diffusion barrier which interrupts the thoron gas entering the 75 mm_ hemispheric chamber (only 0.22% of thoron gas diffuses into the chamber) in order to enhance the discrimination ratio. Results of an interim survey using the new dosemeter suggest that certain types of Japanese wooden houses have a high risk due to thoron and its daughters rather than radon and its progeny.