The light source device and the projection display apparatus include a solid-state light source unit, a dichroic mirror, a fluorescent plate, a first wave plate, and a multi-reflection mirror. The solid-state light source unit emits first and second linearly polarized light rays at a predetermined ratio, the first and the second linearly polarized light rays being orthogonal to each other. The dichroic mirror separates the first and the second linearly polarized light rays, and combines blue light with yellow light. The fluorescent plate emits the yellow light to the dichroic mirror when the fluorescent plate is excited by the first linearly polarized light ray separated by the dichroic mirror. The first wave plate converts the second linearly polarized light ray separated by the dichroic mirror, into circularly polarized light. The multi-reflection mirror reflects the circularly polarized light to the dichroic mirror as the blue light.