An illumination optical system includes: a light source emitting light of a first wavelength; a fluorophore unit; an optical element and a quarter-wave plate between the optical element and the fluorophore unit. The fluorophore unit has a reflection region and a fluorophore region that gives off fluorescent light of a different wavelength than the first wavelength when irradiated by light of the first wavelength. The fluorophore unit is able to move such that the light from the light source sequentially irradiates the fluorophore region and the reflection region. The optical element separates the light of the first wavelength into a first linearly polarized light component and a second linearly polarized light component that is orthogonal to the first linearly polarized light component. The optical element guides the first linearly polarized light component of the light emitted from the light source to the fluorophore unit, and emits the light of the first wavelength that was reflected by the reflection region and fluorescent light that was emitted by the fluorophore region in the same direction.