A light source device includes: a first optical element through which emission light emitted from each of semiconductor laser elements propagates; a first light receiver that receives first propagating light that has propagated through the first optical element; a laser driving controller that controls the semiconductor laser elements; and a measurement circuit that measures a first output value that indicates a received-light intensity of the first propagating light that has been received by the first light receiver. The first light receiver is disposed downstream of the first optical element. The laser driving controller drives the semiconductor laser elements by using a plurality of values of a driving current that are different from each other. The measurement circuit measures the first output value of the first propagating light received by first light receiver for each of the plurality of values of the driving current that are different from each other.