The present invention is made to not only reduce voltage drop when a power supply circuit makes a selection between a power supply voltage from a main power supply and a backup power supply voltage from an auxiliary power supply, but also reduce the power consumption of the auxiliary power supply. The power supply circuit, which is in a semiconductor integrated circuit, includes a measurement circuit, a switch control circuit, and a switch circuit. The switch circuit includes a first switch element and a second switch element. The first switch element is coupled between an output terminal and a first input terminal. The second switch element is coupled between the output terminal and a second input terminal. The measurement circuit operates on the main power supply voltage at the first input terminal and compares the main power supply voltage with the auxiliary power supply voltage.