A power supply device for a supercharging system that includes: a first compressor disposed in an intake air flow passage in which intake air supplied to an engine is circulated and configured to compress the intake air by being driven; an electric motor that is configured to drive the first compressor; a second compressor disposed in the intake air flow passage independently from the first compressor and configured to compress the intake air; and a turbine disposed in an exhaust gas flow passage in which exhaust gas from the engine is circulated and configured to drive the second compressor by being rotated in accordance with the exhaust gas, characterized in that the power supply device comprises: a first power supply circuit including a power storage device having a first capacity and configured to supply electricity of a first voltage to the electric motor; a second power supply circuit including a power storage device having a second capacity smaller than the first capacity and configured to supply electricity of a second voltage higher than the first voltage to the electric motor; and a power supply-switching unit that is configured to supply electricity from the second power supply circuit to the electric motor when an operation of the electric motor starts and thereafter supplies electricity from the first power supply circuit to the electric motor, wherein the power supply-switching unit is an analog circuit that is connected to the first power supply circuit and the second power supply circuit and configured to supply the higher one of a voltage of the first power supply circuit and a voltage of the second power supply circuit to the electric motor in a transient operation period. a first compressor disposed in an intake air flow passage in which intake air supplied to an engine is circulated and configured to compress the intake air by being driven; an electric motor that is configured to drive the first compressor; a second compressor disposed in the intake air flow passage independently from the first compressor and configured to compress the intake air; and a turbine disposed in an exhaust gas flow passage in which exhaust gas from the engine is circulated and configured to drive the second compressor by being rotated in accordance with the exhaust gas, characterized in that the power supply device comprises: a first power supply circuit including a power storage device having a first capacity and configured to supply electricity of a first voltage to the electric motor; a second power supply circuit including a power storage device having a second capacity smaller than the first capacity and configured to supply electricity of a second voltage higher than the first voltage to the electric motor; and a power supply-switching unit that is configured to supply electricity from the second power supply circuit to the electric motor when an operation of the electric motor starts and thereafter supplies electricity from the first power supply circuit to the electric motor, wherein the power supply-switching unit is an analog circuit that is connected to the first power supply circuit and the second power supply circuit and configured to supply the higher one of a voltage of the first power supply circuit and a voltage of the second power supply circuit to the electric motor in a transient operation period. a first power supply circuit including a power storage device having a first capacity and configured to supply electricity of a first voltage to the electric motor; a second power supply circuit including a power storage device having a second capacity smaller than the first capacity and configured to supply electricity of a second voltage higher than the first voltage to the electric motor; and a power supply-switching unit that is configured to supply electricity from the second power supply circuit to the electric motor when an operation of the electric motor starts and thereafter supplies electricity from the first power supply circuit to the electric motor, wherein the power supply-switching unit is an analog circuit that is connected to the first power supply circuit and the second power supply circuit and configured to supply the higher one of a voltage of the first power supply circuit and a voltage of the second power supply circuit to the electric motor in a transient operation period.