Upon generating arc discharge by inputting electric power which changes at a predetermined frequency to the discharge lamp horizontally disposed so as to operate the discharge lamp, the predetermined frequency is a frequency such that the arc discharge is excited substantially in a linear shape by acoustic resonance; and a value of the electric power to be input to the discharge lamp is set so that an input power value per unit inner volume of an arc tube of the discharge lamp falls within a predetermined range which is determined depending on a shape of the arc tube, thereby suppressing a strip-shaped adhesion of a sealed material of the arc tube to a tube wall in the vicinity of a center portion of the arc tube.