PURPOSE: To automatically control the supply quantity of a liquid, by dividing a container into a liquid inlet side and a liquid outlet side by means of a vacuum retaining wall, detecting the liquid level of the liquid inlet side and by controlling the opening of a vacuum adjusting valve in accordance with the detected liquid level.

CONSTITUTION: A vacuum area is defined in a container 2 of a mixer settler 1 between a separating wall 3 dividing the container into a liquid inlet side 2a and a liquid outlet side 2b and a vacuum retaining wall 4 in communication with a vacuum source through a pipe 8 provided with a vacuum adjusting valve 7. A L-shaped transparent pipe 9 is connected to the bottom of the liquid inlet side 2a of the container 2. A light receiver 11 for detecting the transmitted quantity of parallel light rays from a light source 10 detects a liquid level of a vertical portion 9a of the pipe 9, thereby sensing the liquid level of the liquid inlet side 2a. A PID controller 13 controls the vacuum adjusting valve 7, so that, when the liquid level of the liquid inlet side 2a becomes high, the opening of the valve 7 is made larger and, when the liquid level becomes low, the valve opening is made smaller. When the valve opening becomes large, the height of inward liquid entry from the vacuum retaining wall 4 increases and, conversely, decreases when the valve opening becomes small.