To control a flow rate and a liquid control level with average residence time maintained in a transient state by increasing or decreasing a liquid inflow by a prescribed ratio and equalizing a liquid outflow to the liquid inflow when the liquid control level rises or falls by the prescribed ratio.

A liquid outflow of a tank chemical reactor in a mass balance state in which a liquid inflow is equal to a liquid outflow is made constant, and when the liquid inflow is increased or decreased by a prescribed ratio to raise or lower a liquid control level by the prescribed ratio, the liquid outflow is equalized to the liquid inflow to make a mass balance state. Therefore, the whole process of changing the liquid control level is performed with average residence time maintained. Since if only each tank chemical reactor is in a mass balance state, increase or decrease in the liquid control level at an arbitrary point in time is started, the liquid control levels of the plural tank chemical reactors connected in series are changed at the same time.