A light source device includes: first and second laser diodes; a reflector having: first and second reflecting faces configured to reflect a portion of light from the respective first and second laser diodes and to transmit a portion of the light from the respective first and second laser diodes, and first and second exit faces configured to allow the portions of the light transmitted through the respective first and second reflecting faces to exit; and a photodetector including: first and second light receiving element configured to receive light exiting the first and second exit faces, respectively. The reflector is configured such that the light transmitted through the first reflecting face is hindered from exiting the second exit face and the light transmitted through the second reflecting face is hindered from exiting the first exit face.