A power supply device of the present invention includes a switching element and a switch that operate so that when a primary DC voltage source of the device is in a voltage drop state, the power of a secondary auxiliary voltage source thereof is mixed with the power of the DC voltage source in a balanced manner and supplied to a load. This structure can reduce the maxim amount of power storage necessary for the auxiliary voltage source and thus the number of power storage elements necessary for the auxiliary voltage source. Thus, a smaller power supply device can be provided.