A color combining optical system which can suppress occurrence of a thermal problem and increase in size is disclosed. The color combining optical system comprises a first optical element which has a first optical film reflecting light having a first polarizing direction and transmitting light in a second polarizing direction and combines the first and second color lights by the first optical film. In addition, the optical system comprises a polarizing plate which analyzes a third color light. Furthermore, the optical system comprises a second optical element which combines, by a second optical film, the third color light transmitted through the polarizing plate with the first and second color lights combined by the first optical element.