A switching power supply device includes a voltage conversion transformer, a primary-side control semiconductor device, a rectification and smoothing circuit, an output voltage detection circuit, a failure detection circuit, and a switch. The primary-side control semiconductor device generates a driving signal which controls a switching element connected to a primary winding of the transformer. The rectification and smoothing circuit is connected to a secondary winding of the transformer. The output voltage detection circuit detects a secondary-side output voltage of the transformer and transmits a feedback signal corresponding to the output voltage to the primary-side control semiconductor device through an insulated signal transmitter. The failure detection circuit detects a failure on a secondary side of the transformer. The switch cuts off a current flowing to the insulated signal transmitter if the failure detection circuit detects a failure.