To simply and accurately determine the concentration of a water treating agent in thermal equipment upon management of a supply amount of the water treating agent, a method of controlling a supply amount of a water treating agent according to the present invention includes the steps of: supplying a water treating agent containing a scale inhibitor selected from chelating agents to a water-supply system of thermal equipment; collecting a water sample from the thermal equipment; quantitatively determining the scale inhibitor in the collected water sample; and determining a concentration of the water treating agent in the thermal equipment on the basis of a quantitative value of the scale inhibitor.