A light source device includes a housing having a great length along a predetermined direction, a plurality of light emitting elements which are placed in the housing and are arranged along the predetermined direction, and one or more heat dissipation members which are placed in the housing and are thermally connected with the light emitting elements. A first intake port through which air is sucked into the housing is provided in one end in the housing. An exhaust port through which air is discharged to an outside is provided in the other end in the housing. A space where the other side in the predetermined direction faces the heat dissipation member is formed in the housing. A second intake port through which air is sucked into the space from the outside is provided in a side surface between the first intake port and the exhaust port in the housing.