[en] Simultaneous and discriminative measurement of the concentrations of radon and thoron in the indoor atmosphere was performed using a passive monitor with CR-39 track detector. Etch pits on a detector were discriminated with their sizes and shapes for alpha particle energies of 6.0 MeV(²¹⁸Po, ²¹²Bi), 7.7 MeV(²¹⁴Po) and 8.8 MeV(²¹²Po). Discriminating capability was enhanced by use of a degrader mounted on the detector. Etch pits were automatically classified for their sizes and shapes using a computer-aided counting system. Radon and thoron daughters were electrostatically collected onto the degrader surface in order to eliminate alpha particles impinged from the space near the detector surface with reduced energy. A cup for electrostatic collection was designed to prevent the alpha particles emitted from daughters attached on the cup wall and to obtain a flat distribution of daughters collected on the detector surface. The results of the application of this method to thoron-rich fields are described. (author)