A voltage regulator has a reference voltage generator that outputs a reference voltage based on first and second electrical source voltages, an output circuit which generates a predetermined direct-current voltage based on the reference voltage and generates a comparison voltage lower than the predetermined direct-current voltage, and a differential amplifier coupled between the reference voltage generator and the output circuit. The differential amplifier provides a control voltage to the output circuit responsive to a difference between the reference and comparison voltages. The voltage regulator has a voltage adjustment circuit that adjusts the reference voltage responsive to a variation in the first electrical source voltage. The differential amplifier may include a constant-current circuit and an operation current generating circuit. The voltage regulator may include a detecting circuit that detects a variation in the first electrical source voltage and controls the operation current generating circuit responsive thereto.