A power supply device generating source powers to be outputted to an inverter and external supply destinations includes: a first power supply transformer including a first primary winding and a plurality of first secondary windings wound on an outer side of the first primary winding; and a second power supply transformer including a second primary winding electrically connected to one of the first secondary windings and a plurality of second secondary windings wound on an outer side of the second primary winding. The second secondary windings are connected to the external supply destinations. The first secondary windings include an upper gate winding connected to an upper-gate power supply circuit for switching on and off a switching element of an upper arm of the inverter, and the upper gate winding is wound on an outermost side to be separated away from the one of the first secondary windings.