The drop size distribution (DSD) of precipitation particles is a fundamental property for characterizing rainfall. This study statistically clarified the characteristics of DSDs using approximately 10 years of DSD data obtained from a ground-based optical disdrometer in Kumagaya, the eastern part of Japan. The results showed that DSDs tended to maintain their shape even as rainfall intensity (R) increased, and they tended to be distributed in a certain region, i.e., the mass-weighted mean diameter (D_m) ∼ 2.0-3.0 mm and the generalized intercept parameter (N_w) ∼ 2 × 10³–3 × 10⁴ mm⁻¹ m⁻³, of the DSD parameter space defined by D_m and N_w. The quasi-equilibrium shape of the DSDs, which is rarely observed only 16 cases in this study, was likely to be different characteristics between maritime and continental convection. Among them, the contribution to R was large when D_m or N_w was effectively increased with temporal change based on an error analysis. DSD characteristics were also identified by statistically evaluating the relationship between the specific differential phase (K_DP) and R in DSDs for C-band polarimetric weather radar. The results showed that the coefficient of the K_DP–R relation tended to be larger (> 24.0) during the warm season (from May to October) and smaller (< 21.0) during the cold season (from January to April and from November to December) when assuming a temperature of 10 degrees Celsius, whereas the exponent of the relation had no apparent trend. Furthermore, it is likely that the slope parameter, one of the DSD parameters, can be optimized for stronger rainfall events with a nearly same size distribution.