PURPOSE: To form an exact proportionate liquid mixture in accordance with the electrochemical characteristics of fluid by providing the system with a means for setting the set point to indicate conductivity, a means for forming a control signal indicating the difference between the conductivity of the liquid mixture and the set point and a means for minimizing the difference.

CONSTITUTION: A conductivity probe 38 of a foam solution mixture release line 31 executes the conductivity measurement of the solution released by a release device 32 and the measurement signal thereof is sent to a microprocessor 34. The microprocessor 34 compares the measuring value of the conductivity by the conductivity probe 36 of the sample liquid mixture in a sampling reservoir 24 and the conductivity of the liquid mixture composed of the water and foam concentrate actually released in the system, forms the control signal according to the compared value and sends the signal to a control unit 28. The adequate regulation of a mixing control valve 26 is carried out and the difference in the measuring values between the sample solution in the sample reservoir 24 and the solution released from the release device 32 is minimized.