Regarding mineral pollution of the river Watarase, studies were made on the properties of its irrigating quality and also on its harmful effects on the growth and yield of paddy rice plant. The results obtained were as follows : - 1. Surface water of the river Watarase was clear and neutral in its reaction and did not contain copper in fine weather, consequently it was suitable for irrigation. But in rainy weather it became very muddy and floated much sand and mud which contained a considerable amount of copper. This amount of copper, taken into the paday field, caused the crops to suffer from the injurious effect of this metal due to the unfavourable conditions occurred in both chemically and physically. If these floating solid matters were removed from the muddy water, the harmful effect would be expected to be got rid of. 2. Ground water from the river Watarase was good in its chemical composition, but had rather lower temperature even in midsummer compared to that of the surface water. Therefore the water needed to be warmed when it was used for irrigation. 3. Harmful effects on the growth and yield of paddy rice plant were due to not only the excess of Cu ion derived from the floating solid matter in muddy water, but also the unfavourable physical conditions of soils. 4. From the point of view of utilisation for irrigation the water was required, first of all, to be free from the floating solid matters, which caused the harmful effects on the rice plant. For the protecting the field from the shortage of irrigating water, the use of ground water was taken. into consideration.