To continue stable control by grouping, registering, and storing set constants of a transfer function, which needs to be changed on a halfway, in basic transfer function units, and changing settings by groups in the same timing.

Transfer functions K1 and K2 of control elements 1 and 2 for primary delay corresponding to an RC circuit are represented as K1=(T /RC)/(1+(T/RC)) ana K2=1/(1+(T/RC)). In the expressions T is a sampling period. This control system is digitally approximated by backward different approximation, and step response characteristics to this control system are as shown in the figure. Here, when a value C is increased to C' as compared in K1 and K2, K1 and K2 become K1' and K2' respectively and registered by the groups in basic transfer function units. Then when a command for set constant alteration is inputted, the time needed for rewriting is secured in the common bus use timing of a management part CPU and then the settings of K1' and K2' are changed by the groups in the same timing. Consequently, the deviation of constants after a switching point is eliminated and ideal switching is done.