The present invention provides a power supply device for an electric vehicle that allows highly efficient operation of a compressor inverter. A power supply device for motor vehicle 10 has: a main circuit 13 having a power source 11, a DC-DC converter 18, and a main inverter 17 that drives a main motor 12; an auxiliary circuit 15 having an auxiliary inverter 19 that drives an auxiliary motor 14, a first electrical circuit 23 that is connected to the main circuit 13 on a primary side of the DC-DC converter 18, a second electrical circuit 24 that is connected to the main circuit 13 on a secondary side of the DC-DC converter 18, and a connection circuit 30 configured to be capable of selecting one of the first electrical circuit 23 and the second electrical circuit 24 as a path for supplying a direct current voltage to the auxiliary inverter 19; and a control device 16 that controls the connection circuit 30 when the main motor 12 is in power running operation so that switching between the first electrical circuit 23 and the second electrical circuit 24 is performed corresponding to a required voltage of the auxiliary inverter 19.