Even if a DC output voltage of a step-down chopper circuit (3) is lowered from a value in a non-lighted state or a no-load state due to a discharge occurring between conductors (105a) in a cable (104), the turn on detection circuit (26a) can discriminates between lighted and non-lighted states without erroneously determining such a discharge as a discharge in a discharge lamp (4). This allows an operation of the timer (29) to be continued, so that high-voltage pulses are intermittently applied to prevent continuous discharge between the conductors (105) and abnormal heat generation in the cable (104). Further, even if a discharge occurs in an outer tube of the discharge lamp (4), the turn on detection circuit (26a) never erroneously discriminates this discharge as the lighted state. Thus, the lighting device of the present invention can suppress abnormal heat generations in the components and a socket (102).