Abstract: A liquid concentration control and supply apparatus used in a reaction apparatus is disclosed. A control element in the liquid concentration control and supply apparatus controls its operating steps, comprising detecting the mixed liquid concentration in a liquid mixing device, detecting the height of mixed liquid level in the liquid mixing device, determining whether the mixed liquid concentration falls within a lag range referencing to a preset concentration value, and based on the information on the mixed liquid concentration and the mixed liquid level, computing the anticipated height of mixed liquid level after the amount of liquid that needs to be output by the liquid storage device is added to the liquid mixing device and controlling the output of liquid from the liquid storage device so that the mixed liquid concentration falls within the lag range referencing to the preset concentration value.