Continuous stormwater runoff monitoring was conducted in May 1999 and from April 20 to May 24, 2000, at a separate sewer system with 67 ha drainage area. The impervious surface covers about 40 % of the entire drainage area in this study. The monitoring stations were equipped with a direct sampling device from stormwater pipes, a water level sensor, a rain gauge, and a turbidity sensor. The monitoring frequency of the sensors was set at one minute. Two sets of autosamplers with 24 bottles were installed for runoff sampling during initial 8 hours of a rainfall event. Intensive sampling was scheduled within the initial runoff period. Suspended solids were measured as an indicator of pollutants for runoff samples prefiltered with a 2 mm mesh, and furthermore they were classified into fine and coarse fractions with a GF/C filter (1.2 μm pore size) and stainless steel sieve (45 μm mesh). The behaviors of fine particle fractions from 1.2 μm to 45 μm and coarse particle fractions from 45 μm to 2 mm showed clearly different runoff patterns. The fine particle runoff took place in all monitored rainfall events, while the coarse particles tended to be washed off when the rainfall intensity was over a certain level. The fine particle fraction contributed to the total runoff loads rather than the coarse particle fraction in the monitored rainfall events with a small amount of rainfall height and weak intensity.