In order to obtain a constant current circuit which has an excellent constant current property and requires no plural bias circuits, a base of an NPN bipolar transistor (5) and a gate of an N-channel MOS transistor (6) are connected to a first terminal (1) in common. A collector of the transistor (5) is connected to a second terminal (2) and a source of a transistor (6) is connected to a third terminal respectively, while a voltage source (59) is connected between the first and third terminals. An emitter of the transistor (5) is connected with a drain of the transistor (6). Identical bias voltages are supplied to the base and the gate, while a gate-to-drain voltage of the transistor (6) is equal to a base-to-emitter voltage of the transistor (5). Thus, the transistor (6) operates in a pentode region, to serve as a constant current load for the transistor (5).