PURPOSE: To efficiently project an image by obtaining an irradiated area whose longitudinal/lateral ratio corresponds to the aspect ratio of the image to be projected.

CONSTITUTION: This light source device 17 is used for the projector 1 which displays the image by transmitting light through liquid crystal display panels 7, 8, and 9 and projects it forward and a pinion 23 engaging a rack 26 provided to an electric lamp support 24 supporting a tungasten halogen lamp 25 positioned in a reflector through the rear insertion hole 19c of the reflector 19 is rotated by a motor 22 to shift the electric lamp in position relatively to the reflector, thereby obtaining the irradiated area whose longitudinal/lateral ratio which is nearly equal to the aspect ratio of the image displayed on a liquid crystal display panel.