A light source device includes an LD module having a plurality of small light sources such as LDs, a control device selectively controlling On/Off of a plurality of the LDs, a first optical system collecting light emitted from the LDs into one collected light, and a second optical system emitting the collected light directly or through an optical process as source light. The control device maintains a constant amount of luminescence of the light source by controlling a number of the LDs selectively turned on in order to achieve a simple configuration of the light source device and a proper gradation expression of images.