Bases of two grounded transistors are connected with each other to form a current mirror bias circuit. A signal is inputted to the base of one of the grounded transistors. To reduce the influence of β difference between the two grounded transistors by an amplifier to which the signal is outputted from the collector of the one of the grounded transistor, NPN transistors (Q4, Q5) are connected to the base electrodes of the two transistors and their collectors are connected to a power line (Vcc). Thus, the influence of β difference between the two grounded transistors can be reduced while the necessary power-supply voltage is maintained at a low level.