The present disclosure provides a light source device which utilizes polarization properties of light emitted from a solid state light source and uses a low-cost retardation plate having excellent durability and a projection display apparatus which uses the light source device. The p-polarized blue light emitted from a semiconductor laser (21) passes through a condensing lens (23), a lens (26), alens (27), and a first diffusion plate (28) to enters a dichroic mirror (29). A part of the p-polarized blue light passes through the dichroic mirror (29) and the rest p-polarized blue light is reflected by the dichroic mirror (29). The blue light passing through the dichroic mirror (29) is condensed by the condensing lens (23) and passes through a quarter-wave plate (38) to enter a reflection plate (39). The blue light, reflected by the reflection plate to become divergent light, passes through the quarter-wave plate (38) to enter a condensing lens (36) to become parallel light.