PROBLEM TO BE SOLVED: To provide a compact light source device capable of controlling the quantity of outgoing light.SOLUTION: A light source device includes: a lamp light source and light quantity adjusting means for adjusting the quantity of outgoing light by reducing a beam of light from the lamp light source. The light quantity adjusting means includes: a first circular plate provided with a plurality of first light passing holes and disposed on an optical path of the light beam a second rotatable and circular plate provided with a plurality of second light passing holes and disposed parallel to the first plate on the optical path of the light beam and a driving part for rotating the second plate. The second plate is rotated by the driving part, so that the quantity of outgoing light passing through the first and second light passing holes is gradually changed.
