Provided is a light source device capable of effectively cooling a bulb included therein. The light source device includes: a luminous tube having a bulb to seal a pair of electrodes that generates light by electric discharging; a reflector formed into a concave surface shape and configured to emit light from the bulb; and an air blowing device that generates a flow of gas to cool the luminous tube. The light source device includes: a first flow path for jetting out gas along the inner surface of the reflector to a first hemispherical part located far from the installation surface of the bulb; a second flow path for jetting out gas along the inner surface of the reflector to a second hemispherical part located near the installation surface of the bulb; and a movable plate for separating the first flow path and the second flow path from each other to form a part of each flow path, adjusts the amount of gas flowing into each flow path to adjust the ratio of the air flow in each flow path, and gradually narrows the flow path area in the flow path adjusted to increase the air flow in the gas jetting-out direction.