The power supply apparatus comprising a generation unit configured to generate a pulse signal, a memory unit configured to store information set in advance on the driving frequency, and a determination unit configured to determine an error of the input voltage and an error of a load, to a piece of the information stored in the memory unit, a driving frequency detected by the detection unit under a state where a load is connected to the convertor, and based on a result of comparing, to a piece of the information stored in the memory unit, a driving frequency detected by the detection unit under a state where the load is not connected to the convertor.