A lighting device includes at least two lines connected in parallel, a constant current source for applying a constant current to at least one of the at least two lines, an ON/OFF controller, and a voltage detector for measuring a value of voltage flowing in each line of the at least two lines. Each line of the at least two lines is formed by one or more light-emitting diodes and a switch for turning ON/OFF the current flowing in the light-emitting diodes connected in series, and according to the voltage value of each line of the at least two lines measured by the voltage detector, the ON/OFF controller controls the duty ratio of the one or more light-emitting diodes of each line in accordance with the voltage measured.