To reduce the waste of liquid untill multiple liquid are mixed in a prescribed ratio as much as possible even if a plurality of feed passages are different in volume.

A first feed passage 4 of feeding a main agent from a main agent feed source 2 to a mixer 1 is provided with a first opening and closing valve 6 and a flowmeter 7. A second feed passage 5 of feeding a hardener from a hardener feed source 3 to the mixer 1 is provided with a second opening and closing valve 8 and a flowmeter 9. A controller 10 controls each opening and closing valve 6 and 8 so that the main agent and the hardener are fed to the mixer 1 in a previously set prescribed ratio based on signals from each flowmeter 7 and 9 in a mixing stage of a coating material. At this time, the volumes of the feed passages 4 and 5 (the volumes from the opening and closing valves 6 and 8 to the inlet of the mixer 1) are previously memorized in the controller 10. On start of the mixing stage, at first, initial feed operation of feeding the main agent and the hardener through the feed passages 4 and 5 only by the memorized volumes is performed. Thereafter, normal feed operation is performed.