A light source device includes: a substrate having a support face; a lateral wall part disposed on the substrate and having an upper face and inner wall faces, the inner wall faces defining a space; a laser diode located in the space; a first submount having a mounting face bonded to an upper face of the laser diode, and an upper face located opposite the mounting face; a sealing member bonded to the upper face of the lateral wall part and the upper face of the first submount, thereby sealing the space; a heat dissipating block located above the first submount; and a heat conducting member located between the first submount and the heat dissipating block.