Columnar prisms 22 to 24, cross sections of which are parallelograms, are arranged symmetrically on both sides of a columnar prism 21 whose cross section is an isosceles right triangle. The prisms are bonded to their adjacent prisms at polarization separation films D1 to D3 which are dielectric multilayer films. A reflective films is applied on the outermost surface of the outside prism 24. The polarization separation films D1 and D3, which are located in odd-numbered order from the prism 21, transmit incident p-polarized light and reflect incident s-polarized light, the polarization separation film D2 located in even numbered order from the prism 21 transmits the incident s-polarized light and reflects the incident p-polarized light. In order to achieve uniform polarization at the emission surface of a polarization separation/conversion device 20, a 1/2 wave plate C2 is applied on to the emission surfaces of the prisms 22 and 23. The advantages can be achieved in a structure in which the polarization separation film D2 is replaced by a reflective film.