A projection optical system projects onto a projection surface an image ray bundle formed on an image display element, and includes a transmissive optical system positioned on a side of an emission surface of the image display element and having positive power, and a reflective optical system including at least one mirror for reflecting, toward the projection surface, light rays emitted from the transmissive optical system. The transmissive optical system includes at least one positive lens disposed closer to the image display element than an aperture stop, along with first and second positive lenses having a meniscus shape, and a negative lens disposed therebetween, which are closer to the projection surface than the aperture stop is. During focusing, spacing between the first positive lens and the negative lens and spacing between the second positive lens and the negative lens remain unchanged.