A substrate treating apparatus for heating a treating solution formed of a chemical and a diluent, and immersing substrates in the treating solution for treatment. The apparatus includes a treating tank for storing the treating solution, a heater for heating the treating solution, a temperature detector for detecting temperature of the treating solution, a temperature controller for controlling the heater so that a detected temperature of the treating solution reaches a set temperature, a supplementing device for supplement the treating tank with the diluent, a concentration detector for detecting concentration of the treating solution, and a concentration controller, operable only when the temperature detector detects the treating solution being in a temperature range close to the set temperature, for controlling the supplementing device to adjust an amount of the diluent supplemented so that the concentration of the treating solution detected by the concentration detector becomes slightly higher than a boiling-point concentration corresponding to the set temperature of the treating solution.