Disclosed are a semiconductor integrated circuit including first and second supply terminals; an input voltage selection circuit including a power-on reset circuit, an input voltage detection circuit, a control circuit and a power-on reset auxiliary circuit; and first and second power supply switches, and its operating method. When the above detection circuit detects the supply of both first and second power supply voltages to both supply terminals upon completion of a power on reset operation of the reset circuit, the control circuit controls either of both power supply switches, having a high priority of preset priorities, to an on state. After its control, the auxiliary circuit detects a power failure in the power supply voltage supplied to the high-priority power supply switch. The reset circuit performs another power on reset operation. The control circuit controls the power supply switch having a low priority of the priorities to an on state.