A light source device for an endoscope system comprises a light source unit, which collects light with a lens and allows the collected light to enter a light guide provided to an endoscope. The light source device comprises a receiving unit and a lens barrel (movable unit). The receiving unit receives insertion of the light guide. The receiving unit is attached to the lens barrel. The lens barrel moves in a direction perpendicular to a direction of the insertion of the light guide, in accordance with a force received by the receiving unit from the light guide. The light source unit applies collimated light to the lens. The lens is attached to the lens barrel and moves together with the receiving unit and the lens barrel, and makes at least a part of the collimated light enter the light guide.
