PURPOSE: To cope with variation in controlled variable by selecting and inputting a physical quantity as a measured value, a controlled variable, and a physical quantity as a set value to a controller and selecting and setting a function corresponding to the physical quantities out of registered functions.

CONSTITUTION: Functions f(X) registered in function circuits 22a and 22b formed in the controller prescribe the relations between measured values and set values of a water flow system; and the relation between the water level W of a water storage tank when the water level 'W' is regarded as a measured value and a flow rate set value Qset of the inflow to the water storage tank is defined in the function circuit 22a and the relation between the flow rate Q in a conduit when the flow rate Q is regarded as a measured value and a pressure set value Pset of supplied pressure is defined in the function circuit 22b. A selecting circuit 23 provided between an input circuit 21 and the function circuits 22a and 22b, when inputting the physical quantity as the measured value, a controlled variable, and the physical quantity as the set value to the controller prior to control, selects and sets the function corresponding to the selectively inputted physical quantities out of the previously registered functions.