A base compound supply passage and a hardener supply passage supply a base compound and a hardener, respectively, fed under pressure from first and second color changing valve mechanisms to a coating gun. The base compound supply passage and the hardener supply passage have respective first and second gear pumps, and first and second pressure control valves for controlling the base compound and the hardener to be supplied under given pressures to the first and second gear pumps. Two liquids, i.e., the base compound and the hardener, can be supplied reliably at desired rates to the coating gun, and mixed and discharged at a desired mixing ratio.