In order to provide an optical system to be installed in a projection system which is capable of allowing a patient to view a clear image in a diagnostic imaging apparatus, the optical system according to the present invention includes a Fresnel lens having a base material and a plurality of annular sections arranged around a center axis on a reference surface of the base material, and a deflection element configured to deflect light from the Fresnel lens. In a cross section including the center axis, an inclination angle of the annular sections to the reference surface changes asymmetrically between from a middle point of one and other ends of the Fresnel lens to the one end and from the middle point to the other end.