A control circuit U1 comprises four PMOS transistors MP1-MP4 and receives a voltage Vn and a voltage Vss. MP1 and MP3, and, MP2 and MP4 are respectively connected in series between power supply Vdd and a fixed voltage Vss. Gate terminal of MP2 is connected to Vss. Reference current and its copy current F1 respectively flow through NMOS transistors M1 and M2, of which respective source terminals are connected to Vss. Gate width of M2 is a quarter of that of M1. Drain terminal is connected to the gate terminals of MP1 and MP2. Node between source terminal of MP2 and drain terminal of MP3 is connected to gate terminal of MP1, and node between source terminal of MP2 and drain terminal of MP4 is connected to gate terminal of MP2. The control circuit U1 controls gate terminal voltage of M1 to make an overdrive voltage of M1 becomes Vn.