PURPOSE: To accurately keep a residual chlorine content at an objective level on a water-examination point after injection of chlorine into a civil water-purification plant, by injecting chlorine in accordance with a chlorine-injecting pattern classified and prepared according to seasonal and weather conditions.

CONSTITUTION: A rate of chlorine to be fed from a chlorine injector 14 is stored in a memory device 1. A measured value from a pyrheliometer 5 is converted into a rate of chlorine consumpition caused by solar radiation in an operational circuit 8, and stored in a pattern memory device 11. A chlorine consumption other than that caused by solar radiation is computed by another operational circuit 4, and stored in a memory device 7. A pattern-operating circuit 10 calculates a chlorine-injection ratio from an objective residual chlorine value set in an interaction device 3, the rate of chlorine consumption caused by solar radiation and the rate of chlorine comsumption from the memory device 7. The calculated value pickes up a pattern preset by the interaction device 3 from a memory device 6, and is corrected by a correcting circuit 9 using an injection ratio defined by the picked-up pattern.