A semiconductor integrated circuit includes a constant-voltage circuit that outputs a constant voltage and a constant-current circuit. The constant-current circuit includes a first transistor. A voltage input as an input voltage to a first gate electrode of the first transistor is equal to the node voltage at an intermediate node in a connection path between the output of the constant-voltage circuit and an external resistor. A source region of the first transistor is connected to a fixed power supply, and a drain region of the first transistor is connected to the external power supply via a load. In the first transistor, a constant current can be made to flow between the source region and the drain region based on the input voltage. A constant-current drive system in which the semiconductor integrated circuit and the external resistor have been mounted on a circuit board is also constructed.