A lighting power source according to an embodiment includes a rectifying circuit, a smoothing capacitor, a reference voltage generating circuit, and a DC-DC converter. The rectifying circuit rectifies an AD voltage input thereto. The smoothing capacitor smoothes an output from the rectifying circuit. The reference voltage generating circuit generates a reference voltage on the basis of at least any one of an output voltage of the rectifying circuit and a voltage from the smoothing capacitor. The DC-DC converter includes an output element and a constant current element, and converts the voltage of the smoothing capacitor. The output element receives a supply of a voltage of the smoothing capacitor, oscillates by performing a switching operation which repeats an ON state and an OFF state when the reference voltage is relatively high, and continues the ON state when the reference voltage is relatively low.