To maintain a requested raw material composition at all times by increasing and decreasing the set values of the flow meters of respective raw materials when the change of the integrated value of respective flow rates is generated and correcting the deviation of the integrated value.

After inputting a product total amount and the ratio of the respective raw materials by a CRT 11 and a keyboard 12 from an operator's console before starting mixing control, a mixing start command is inputted, the arithmetic operation of a DDC arithmetic block 4 is started and the set values to respective raw material flow rate adjustment meters 5, 6 and 7 are outputted to respective flow rate control operation ends 8, 9 and 10. Then, the respective flow rate control operation ends 8, 9 and 10 are operated, the raw materials of respective raw material tanks 1-3 are made to flow, the signals of the respective flow meters 1E-3E are inputted to the DDC arithmetic block 4, the set values when the change of the integrated value of the respective flow meter 1E-3E is generated are increased and decreased and the deviation of the integrated value is corrected. Thus, a requested mixing composition is maintained at all times from the start of mixing to the end.