A first current flowing through a first resistance is determined by a series connection between a first resistance and a transistor which is short circuited between the gate and the drain. Further, a second current flowing through a second resistance is determined by a series connection between a resistance and two or more transistors each having a short circuit between the gate and the drain. By drawing the second current from below the first resistance, a current fed through a reference transistor is established to be equal to (the first current)-(the second current). The second current starts flowing when the source voltage is equal to or greater than the summed values of the voltage drops between the gate and the source of the two or more transistors. Therefore, the second current becomes zero when the source voltage is lower than a predetermined value. In this manner, a sufficient amount of current is output when the source voltage is relatively low, and the constant current output when the source voltage is high can be controlled so as not to be unnecessarily large.