A discharge lamp lighting device includes: a power control circuit outputting DC current; an AC conversion circuit being supplied with the DC current and generating and outputting discharge-lamp driving AC current by inverting the polarity of the DC current at a predetermined time; and a control unit performing a duty ratio control process of controlling a duty ratio on the basis of a polarity inversion time of the discharge-lamp driving AC current on the AC conversion circuit and performing an interval current control process of controlling a current value of the DC current every polarity inversion time interval on the power control circuit. Here, the control unit performs the interval current control process so as to control a difference between an average value and a maximum value of the DC current of each polarity inversion time interval on the basis of a difference between the duty ratio and a predetermined reference duty ratio.