A light source device, including an alternating current high-pressure mercury lamp having a pair of electrodes opposite each other with a spherical light-emitting part having at least 0.15 mg/mm.sup.3 of mercury and halogen at 10.sup.-6 .mu.mol/mm.sup.3 to 10.sup.-2.mu.mol/mm.sup.3, and cylindrical hermetically sealed parts extending from both ends of the light-emitting part. External leads protrude from the ends of the light-emitting part for receiving alternating current. A reflecting mirror having a concave reflecting part surrounds the light-emitting part and a cylindrical neck part. An adhesive for securing the mercury lamp and the reflecting mirror fills a periphery of the hermetically sealed parts of the mercury lamp. The distance L2 (mm) from a center in between the electrodes to the adhesive is set at a value of one of 16 mm or less and 20 mm or greater.