A concentration monitoring method/apparatus measures electrical conductivity of dialysate prepared by mixing liquid A, liquid B, and dilution water and monitors concentration by comparing the electrical conductivity with a predetermined reference value. First, the method/apparatus determines a ratio (straight line A) of changes in the electrical conductivity of the dialysate to concentration changes of liquid A alone as well as a ratio (straight line B) of changes in the electrical conductivity of the dialysate to concentration changes of liquid B alone. Next, the method/apparatus determines the electrical conductivity of the dialysate to be newly prepared, in accordance with concentration of liquid A and concentration of liquid B in the dialysate to be newly prepared, based on the ratios of the electrical conductivity of the dialysate to the determined concentration changes of liquid A and liquid B and sets the reference value based on the determined electrical conductivity. This makes it easy to set the electrical conductivity according to concentration of the dialysate to be prepared and allows concentration changes to be made even during dialysis treatment.
