The present invention provides a constant current circuit capable of operating on a lower power supply voltage. If a power supply voltage VDD is higher than a sum of a drain-source voltage of a depletion type NMOS transistor (10) Vds10 and a gate-source voltage of the NMOS transistor (15) Vgs15, the constant current circuit can operate. As the power supply voltage VDD of the constant current circuit, a voltage with addition of one drain-source voltage and one gate-source voltage is needed, otherwise a voltage with addition of one drain-source voltage and two gate-source voltages is not needed, thus the lowest operating power supply voltage of the constant current circuit is reduced.