A polarity-reversal control circuit gives a polarity-reversal order so that the polarity of an inverter is inverted each time the polarity-reversal timing notified by a signal is recognized. The number of times of consecutive appearances of a normal polarity-reversal interval period, in which an interval of adjoining polarity-reversal timings is equal to or less than an upper limit of a polarity-reversal interval desirable for a discharge lamp, is counted, and if the unusual polarity-reversal interval period, in which an interval of the adjoining polarity-reversal timing is longer than the upper limit of a polarity-reversal interval, appears, Npm is memorized as the maximum of the number of times of consecutive appearances and after that, when the polarity-reversals signal in which the number of times of consecutive appearances becomes equal to Npm is received, it is regarded as a start of an unusual polarity-reversal interval period, and additional polarity-reversal order is given.