An optical device which is disposed between a light source section of a display apparatus and a projection target member, includes a first luminous flux diameter enlargement element in which a first light-transmissive layer and a first partially reflective layer are alternately laminated in a first direction, and a second luminous flux diameter enlargement element in which a second light-transmissive layer and a second partially reflective layer are alternately laminated in a second direction intersecting the first direction. The first partially reflective layer and the second partially reflective layer are formed from dielectric multilayered films that have the same film configuration. In addition, the optical device includes a retardation element that is formed from a ½λ retardation plate between a first emission surface of the first luminous flux diameter enlargement element and a second incidence surface of the second luminous flux diameter enlargement element.