PURPOSE: To prevent variation in power line by providing a feedback control circuit which monitors and compares the voltage value of the power line with a reference voltage and provides feedback to the power line so that the power line has a constant value.

CONSTITUTION: An operational amplifier 10 as the feedback control circuit operates by the power source of a system different from a +5V line. An output buffer 5 operates at the same time and when a current is supplied from the +5V power source, the voltage of the power line 6 drops, so that the operational amplifier 10 detects the voltage drop of the power line 6. In concrete, the voltage of the power line 6 is compared with the reference voltage +5V. The operational amplifier 10 supplies a current to the power line 6 according to the difference between the reference voltage +5V and the voltage of the power line 6 to compensate the voltage drop of the power line 6. Consequently, the voltage of the power line 6 is controlled to be equal to the reference voltage +5V, and the output buffer 5 operates to suppress the voltage drop of the power line 6.