To calculate a target flow rate from measured quantities of successive processes at optimum time intervals and to control the quantity of water received in every unit time to a target water quantity by calculating a control command according to the cumulative quantity of measured quantities.

The state of a received water flow rate is measured by a flowmeter 2 and stored in an actual result value storage table 20, and a cumulative actual result value 19 is calculated. An operation cycle calculation device 100 indexes a set command 13 and the cumulative actual result value 19 to generate an elapsed time counter 14 and a command operation frequency counter 16, and sets cycles up to next command operation in a command operation cycle 15. A command operation unit 200 calculates a cumulative command 21 and a command from the set command 13 and cumulative actual result value 19, the elapsed time counter 14, and command operation cycle 15, and sets them in a command storage table 22. Here, the command operation unit 200 calculates a command for the set command 13 in consideration of the past cumulative actual result value and the remaining time up to a final control period.