PURPOSE:To realize the automatic mixing of a liquid A and a liquid B to thereby realize the continuous feeding thereof by supplying a liquid B metered in a liquid B storage tank to a liquid A mixing tank where it is mixed with an aqueous sodium silicate solution to prepare a liquid A. CONSTITUTION:A prescribed amount of water is supplied through a supply pipe 7 to a liquid B mixing tank 40 and an electrode 42 is operated and stopped. An agitator 41 is operated and a water-soluble inorganic acid (acidic salt) or organic acid is charged to the tank 40 and mixed to give a liquid B. Upon receipt of a signal from a liquid B storage tank 5, the agitator 41 is stopped, and the liquid B is fed into a reservoir 50 of the tank 5. Part of the liquid B is injected through a withdrawal pipe 53, and part of it is fed into a metering tank 80 by a pump 81. Upon receipt of a signal from a liquid A mixing tank 2, the pump 81 is stopped, and the liquid B in the tank 80 is supplied through a supply pipe 83 to the tank 2, where it is mixed with an aqueous sodium silicate solution supplied through a supply pipe 6 to prepare a liquid A. The liquid A is then fed into a liquid A storage tank 3 and injected through a withdrawal pipe 33.
