PURPOSE: To provide a circuit which can be switched with the normal calculation maintained by adding newly to a bumpless circuit a combination of an unbalance correction circuit at the time of switching and a one-shot tracking circuit.

CONSTITUTION: When, during the one element control in the control of the water level of a deaerator, a switching device 3 is closed to (a) side and the tracking signal 7 is made to be the deviation between the flow rate of supply water and the flow rate of condensate, then the output of PI calculator 1 makes one-shot tracking to the tracking signal 7 by a contact point 5 during the switching from one element to three elements so that the deviation becomes zero, and since the output of PI calculator 2 on the side of three elements in the initial stage is equal to the output of the PI calculator 1 by the tracking during the one element control, the output 9 is the same even if the output 9 is switched to the three element side, unless any one of water level measurement, flow rate of supply water, and flow rate of condensate changes or unless water level setting is changed and the calculation switching with a constant output 9 can be made. During the three element control, if the switching device 3 is closed to the (b) side and the tracking signal 7 is the output 9, the output is equal to the output 9 because the output of the PI calculator 1 makes one-shot tracking to the tracking signal 7 by the contact point 5 during the switching from three elements to one element.