The National Institute of Radiological Sciences (NIRS) has developed passive radon ((222)Rn)-thoron ((220)Rn) discriminative detectors for a large-scale survey and has established a thoron chamber to calibrate such detectors. In order to establish quality assurance and quality control for the (220)Rn measurement at NIRS, intercomparison studies have been carried out. The intercomparisons using a scintillation cell method, which has been used as a standard for (220)Rn measurement at NIRS, were conducted at New York University (NYU, USA) and Physikalisch-Technische Bundesanstalt (PTB, Germany). As a result, it was found that the result from the NIRS was in good agreement with that from the NYU. On the other hand, it was observed that the relative discrepancy between the (220)Rn concentrations from the NIRS and PTB monitors was, on average, >50 %. Using the NIRS (220)Rn chamber, the international intercomparison experiment for passive (220)Rn detectors started in 2008.