A light source device according to the present disclosure includes a light source unit, a collimator, a first step mirror, a second step mirror, a condenser lens, and an optical fiber. The first step mirror reduces a plurality of parallel beams in a row direction by narrowing a column interval between the plurality of parallel beams. The second step mirror reduces the plurality of parallel beams in the column direction by narrowing a row interval between the plurality of parallel beams. The number of steps of the first step mirror and the number of steps of the second step mirror are different from each other, and a width of each of the plurality of mirror surfaces of the first step mirror and a width of each of the plurality of mirror surfaces of the second step mirror are different from each other.