Provided is a constant current circuit capable of supplying a stable constant current. Even when K values of NMOS transistors vary due to manufacturing fluctuations in semiconductor devices, a voltage generated across a resistor is always a threshold voltage difference between the NMOS transistors, and thus hardly varies. Even when the K values of the NMOS transistors vary due to a change in temperature, the voltage generated across the resistor is always the threshold voltage difference between the NMOS transistors, and thus hardly varies.