A light source device includes a light source bank that accommodates a plurality of semiconductor light-emitting elements, and a light source bank holder that mounts the light source bank on a first surface and includes a cooling mechanism provided on a second surface that is a back surface of the first surface. The cooling mechanism includes a heat dissipating part provided at a position corresponding to the light source bank with the light source bank holder interposed therebetween, a flow path cover that covers the second surface so as to accommodate the heat dissipating part, an inflow terminal into which a coolant flows, the inflow terminal being provided on the flow path cover, an outflow terminal from which the coolant flows out, the outflow terminal being provided on the flow path cover, so that a flow path by the heat dissipating part is formed between the outflow terminal and the inflow terminal, and a first flow path that is smaller in resistance than the flow path formed in the heat dissipating part and is formed between the inflow terminal and the heat dissipating part.