The invention provides a switching power supply device and a semiconductor device. The switching power supply device comprises a transformer, a switching element, an output circuit, a feedback signal generating circuit and a control circuit, wherein the transformer comprises a primary winding, a secondary winding, and an auxiliary winding; the switching element is connected in series with the primary winding; the output circuit is configured to generate power for output according to electric power transferred from the primary winding to the secondary winding along with switching actions of the switching element; the feedback signal generating circuit generates a feedback signal during a secondary side conduction period according to an auxiliary winding voltage generated through induction by means of the auxiliary winding within the secondary side conduction period in which a current flows through the secondary winding; the control circuit drives the switching element according to the feedback signal, and comprises a voltage detection circuit and a stop circuit; the voltage detection circuit determines whether the auxiliary winding voltage exceeds a threshold value in the secondary side conduction period in which the current flows through the secondary winding; and the stop circuit stops the switching action according to a signal of the voltage detection circuit.