Abstract: A switching-mode power supply is configured so that two power supply lines are connected with each other via a series circuit consisting of a main switching element, a switch, and a transformer. The switching-mode power supply further includes a ripple voltage detecting circuit for detecting that a ripple voltage in one power supply line has a predetermined amount or more when a half short-circuit occurs in the main switching element. When the ripple voltage detecting circuit detects that the ripple voltage has a predetermined amount or more, the switch is opened so as to block supply of a current (supply of a voltage) to the main switching element. This allows the switching-mode power supply to avoid, with a simple circuit configuration and low costs, being an unstable state when the half short-circuit occurs in the main switching element.