A switching power supply comprising: a control circuit which is provided at a primary side and controls a switching element based on a value which is fed back from a secondary side; a transformer; a secondary side rectifier element which rectifies voltage which is supplied from the transformer; and a secondary side smooth element which smoothes voltage which is rectified by the rectifier element, wherein voltage which is smoothed by the smooth element is supplied to an amplifier and voltage which is smoothed by the smooth element is fed back to the control circuit, further comprising: a diode which is connected between the smooth element and the amplifier in series.