The optical system introduces light from a display element displaying an image to an observer. The optical system includes in order from the display element toward the observer, a first phase plate, a semi-transmissive reflective surface, a lens, a second phase plate, and a polarization beam splitter configured to reflect a first linearly polarized light and transmit a second linearly polarized light whose polarization direction is orthogonal to that of the first linearly polarized light. The semi-transmissive reflective surface has a transmittance smaller than a reflectance thereof.