A lighting device comprising: a DC generation unit (AC, DB, C11) comprising: a rectifying circuit (11, DB) which is configured to input an AC power supply (10, AC) and to output a DC voltage; and a bypass capacitor connected to an output of the DC generation unit; a buck converter connected to the output of the DC generation unit, the buck converter comprising: at least one normally-on type switching element; a diode connected to the switching element; and an inductor connected to the switching element, the inductor being configured to pass through an increased current in turning on the switching element; a semiconductor light emitting element (LC, LD) connected to the inductor, the semiconductor light emitting element being configured to be lit by the DC voltage generated by the buck converter; and a driving control unit connected to a gate of the switching element, the driving control unit being configured to cause the switching element to perform an off operation by applying a negative potential of a forward voltage between a source and the gate of the switching element, and to generate the forward voltage by a current passed through the semiconductor light emitting element characterized by the buck converter comprising a constant-current unit which is a constant current diode having an anode terminal and a cathode terminal, the anode terminal being connected to a source terminal of the switching element, the cathode terminal being connected to the inductor and the diode, wherein the constant current diode is configured to increase voltage (VCCM) in a pulse shape between the anode terminal and the cathode terminal when the increased current reaches a constant current value of the constant-current unit, because the current passed through the inductor is further increased, so that the switching element turns off. a DC generation unit (AC, DB, C11) comprising: a rectifying circuit (11, DB) which is configured to input an AC power supply (10, AC) and to output a DC voltage; and a bypass capacitor connected to an output of the DC generation unit; a buck converter connected to the output of the DC generation unit, the buck converter comprising: at least one normally-on type switching element; a diode connected to the switching element; and an inductor connected to the switching element, the inductor being configured to pass through an increased current in turning on the switching element; a semiconductor light emitting element (LC, LD) connected to the inductor, the semiconductor light emitting element being configured to be lit by the DC voltage generated by the buck converter; and a driving control unit connected to a gate of the switching element, the driving control unit being configured to cause the switching element to perform an off operation by applying a negative potential of a forward voltage between a source and the gate of the switching element, and to generate the forward voltage by a current passed through the semiconductor light emitting element a rectifying circuit (11, DB) which is configured to input an AC power supply (10, AC) and to output a DC voltage; and a bypass capacitor connected to an output of the DC generation unit; a buck converter connected to the output of the DC generation unit, the buck converter comprising: at least one normally-on type switching element; a diode connected to the switching element; and an inductor connected to the switching element, the inductor being configured to pass through an increased current in turning on the switching element; a semiconductor light emitting element (LC, LD) connected to the inductor, the semiconductor light emitting element being configured to be lit by the DC voltage generated by the buck converter; and a driving control unit connected to a gate of the switching element, the driving control unit being configured to cause the switching element to perform an off operation by applying a negative potential of a forward voltage between a source and the gate of the switching element, and to generate the forward voltage by a current passed through the semiconductor light emitting element characterized by the buck converter comprising a constant-current unit which is a constant current diode having an anode terminal and a cathode terminal, the anode terminal being connected to a source terminal of the switching element, the cathode terminal being connected to the inductor and the diode, wherein the constant current diode is configured to increase voltage (VCCM) in a pulse shape between the anode terminal and the cathode terminal when the increased current reaches a constant current value of the constant-current unit, because the current passed through the inductor is further increased, so that the switching element turns off.