To provide a power supply apparatus having high control resolution of an output power. A PWM signal generating circuit includes: a non-inverting element 31, and an inverting element 32; and further includes a counter 11 for performing count operation in response to rising of a clock signal, a counter 12 for performing count operation in response to falling of the clock signal, comparison circuits 21, 22, and a multiplexer 20. These circuit elements are controlled by PWM control means 10. As another circuit element, the PWM signal generating circuit includes a logical sum element 33. The PWM signal generating circuit serves to arbitrarily change both period and logic "H" time of a PWM signal to be outputted at a time interval which is one half of the clock period. Thus, there is provided a power supply apparatus in which resolution of a PWM signal has been improved within a broad duty range, and fine control of an output power has been performed within a broad output power range.