We analyzed the degree of pollution of the water in irrigation ponds in part of Kagawa prefecture and simulated the amount of nitrogen in water to evaluate the beneficial effect of dredging and excavation of the ponds. The average values of TN (Total Nitrogen), COD (Chemical Oxygen Demand) and TP (Total Phosphorus) throughout the period of analysis in 3 ponds exceeded the criteria values of irrigation water for paddy fields and the average values of the pH in all the ponds were also above those of the criteria. Moreover, a considerable degree of eutrophication was observed in 2 of 3 ponds which showed a high average chemical index. In the case of the pH, COD, TN and TP, 90, 58, 50 and 49% of all the water samples showed values above those of the criteria, respectively, while in the case of DO (Dissolved Oxygen), SS (Suspended Solids) and EC (Electric Conductivity), a small number of samples showed values exceeding those of the criteria. Simulation showed that the concentration of nitrogen in the water decreased by 25% and 40% with excavation, resulting in a total volume about 1.5 times as large as that presently. Simulation also showed that the TN concentration of 1.23 mg/L decreased to 1.13 mg/L, indicating that the TN concentration could be reduced by 10% without excavation, should the upper pond be excavated.