Inner structures of organized snow cloud systems associated with a polar-low were investigated with an instrumented aircraft (Gulfstream-II) on 27 January 2001. The echo areas were circular/spiral in shape and quasi-stationary, although they were weak in intensity. The aircraft flew four legs at different heights (10.2km 3.6km 1.5km, 0.3km) along the longitude line of 137.5E, and two legs at different heights (10.2km and 1.5km) along the latitude line of 37.66N. In-situ observations of airflow, thermodynamic and microphysical fields in/around the cloud system were made as well as measurements of reflectivity and Doppler velocity with a w-band cloud radar and dropsonde sounding. The cloud system had the horizontal scale of 100 - 200 km and vertical scale of 3 - 4 km. Characteristic wind fields were detected below the height of 1.5km. Remarkable cyclonic wind patterns were found at the height of 0.3km, in which the easterly winds with a speed greater than 10m/s were on the northern side and westerly winds on the southern side. The warm core was found in the central/southern part of the polar-low, where equivalent potential temperature at the height of 1.5km is higher by 2-3 degrees than its surroundings. Excess vapor produced by the polar-low circulation was consumed by the depositional growth of ice crystals. Those ice crystals were seeded from an upper cloud deck associated with a synoptic-scale low that had passed on the Pacific side of the Japan Islands. Cloud droplet regions were spatially and temporally limited and their water contents were 0.1 - 0.2 g/m3 at most.