Provided is a constant voltage control apparatus capable of reducing feedback lines. In the constant voltage control apparatus, a first amplifier amplifies an output voltage applied to a pair of output terminals. A second amplifier amplifies a voltage drop between one of the output terminals and a load terminal connected thereto. A third amplifier amplifies output voltages from the first and second amplifiers, respectively. Comparing means receive an output voltage from the third amplifier and a reference voltage, and output an output voltage which makes a sum of the output voltage and the reference voltage of the third amplifier become zero. A triac drive unit amplifies the output voltage from the comparing means to control a triac installed on a power line that is connected with the first output terminal.