In a light source apparatus, an alternating current is supplied to a high pressure discharge lamp from a power supply apparatus to light the lamp. The arrangement state of the lamp is detected by a detection circuit. When the lamp is horizontally arranged, electric energy, whose flow direction is from one electrode of the lamp to the other electrode, is set to be approximately the same as electric energy, whose flow direction is from the other electrode to the one electrode. When the lamp is vertically arranged, while a cycle, which is the same as a polarity change cycle in the horizontal arrangement, is maintained, electric energy, which flows from an upper electrode to a lower electrode, is set to be smaller than electric energy, which flows from the lower electrode to the upper electrode.