River pollution by pig farm waste and self-purification were investigated at Ishiuchi river in Hiroshima Prefecture. Following results were obtained. 1) BOD value of pig farm waste water discharged to Ishiuchi river was x^-=4029 (±2059) ppm. Volumes of discharged waste at point No. 2 were 0.002-0.005m³/sec, and the amounts of flow of Ishiuchi river at point No.2 were 0.03-0.21 m³/sec. Self-purification in this river was consisted of three steps : BOD removal by sedimentation, by biochemical process and oxidation of nitrogen compound. 2) Relationship between the amount of flow and self-purification in the river was observed in strongy polluted area. During the rainy season, effect of BOD removal might be raised as compared with that of draught season. Main factor of this phenomenon might be sedimentation of SS. 3) After the completion of SS sedimentation, self-purification was not affected by the amount of flow of the river. 4) Velocity of the BOD removal especially by biochemical process, which was one of the self-purification process in the river, was accerleated up to show the highest value after almost completed sedimentation of easily precipitable SS in strongly polluted area. 5) It is reasonable that self-purification in the river polluted with organic waste should be considered from the standpoint of oxygen balance. According to this consideration, rate of BOD removal in waste being discharged to Ishiuchi river might be required to be 90%.