We paid attention to relationship between farming activities and changes in drainage water quality from a paddy fields area, especially in land preparation period for transplanting of paddy. Drainage water quality during land preparation period became highest concentration of all items through a year. In 2006, average water qualities in the period were 7.9 mg/L in T-N, 1.6 mg/L in T-P, 15.3 mg/L in TOC, and 372 mg/L in SS. In addition, maximum values in the period were 36.2 mg/L in T-N, 12.3 mg/L in T-P, 83.2 mg/L in TOC, and 3020 mg/L in SS. The changes in water quality moved on with land preparation by tractor and drainage gate operation on each paddy field by farmers. Moreover, relatively high concentrations of drainage water were also observed during non-irrigation period. As no agricultural activities were done during the period, it is considered that residual nutrient in soil will infiltrate by rain water and flow to the drainage channel.