To provide a thin rear projection optical system of high performance on which a three-plate illuminating system can be mounted.

A projection optical system for obliquely projecting an image of a panel display surface (I1) on a screen surface (I2) is provided with a refracting lens group (GL), an optical path bending mirror (M1), a power mirror (M2) having a negative power, and a counter mirror (M3) approximately parallel with the screen surface (I2). The projection optical system is approximately telecentric to the panel display surface (I1) side, and the panel display surface (I1) which has the normal parallel with an optical axis (AXG) of the refracting lens group (GL) satisfies conditional formula 0.2<La/Lb<5 where La is the distance between the image center of the panel display surface (I1) and the screen surface (I2) in the normal direction of the screen surface (I2) and Lb is the distance between the image center of the panel display surface (I1) and the reflection surface of the counter mirror (M3) in the normal direction of the reflection surface of the counter mirror (M3).