A switching power supply includes an insulated transformer, a full-bridge circuit which converts input DC power into AC power and outputs the AC power to a primary-side coil, an output circuit which converts AC power input from secondary-side coils into DC power and outputs the DC power, and a control circuit which controls the full-bridge circuit by using a phase shift method based on voltage output by the output circuit. When a measured value of the amount of phase shift determined from at least one of the voltage and current output by the output circuit is smaller than a theoretical value of the amount of phase shift corresponding to the switching power supply operating in a continuous current mode, the control circuit prolongs dead time used in the full-bridge circuit in accordance with the difference between the measured value and the theoretical value.