To blend accurately a chemical concentrate with a diluent liquid by forming a closed loop for a flow duct from a tank, providing a pair of concentration sensors in a branch path on half way of the flow duct, comparing two concentration index values and controlling an adding valve.

A diluent liquid such as water is contained in a tank 10 and is used for blending, and the blended liquid is circulated continuously by forming a closed loop with a tank 10 and a flow duct 19. A supply source 22 of liquid chemicals forms a closed loop with chemicals adding valve 31 and a starting flow duct 19. And on half way of the flow duct 19 branch paths 19.4, 19.5 parallel to each other are provided, and the concentrations of the blended liquid flowing these paths are detected by a pair of dielectric constant type chemical concentration sensors 37, 38. The concentration data outputted from the sensors 37, 38 are compared and their correspondence is checked, the chemicals adding valve 31 is controlled according to the detected chemical concentration of the blended liquid so as to keep the concentration of the blended liquid to be constant always. Thus by improving measurement accuracy of electric conductivity, the blended liquid concentration can be controlled smoothly to the desirable chemical concentration.