A reference current source includes first and second semiconductor elements, a conversion resistor, an amplifier, and a current mirror circuit. The first semiconductor element includes a single diode or a single transistor, and the second semiconductor element includes diodes or transistors connected in parallel. The conversion resistor converts forward voltages of the first and second semiconductor elements or a differential voltage between the first and second semiconductor elements into a converted current. The amplifier has first and second input terminals. The first semiconductor element is connected between the first input terminal and a ground, and the second semiconductor element and the conversion resistor is connected between the second input terminal and the ground. The current mirror circuit outputs a reference current corresponding to the converted current. The conversion resistor has a temperature coefficient being on a level with a temperature coefficient of the differential voltage.