Provided are a color separating and combining prism and an optical system using the same for a projector. A color separating and combining prism (DP) includes dichroic films (B, R) that reflect the light of the wavelength range of either a first or third range from among the first wavelength range of a blue region, a second wavelength range of a green region, and a third wavelength range of a red region, and pass the light of the remaining two adjacent wavelength ranges, and at a specific angle of incidence, the average reflectance in the two wavelength ranges which the films pass is lower than the wavelength average reflectance of the wavelength region sandwiched by the two wavelength ranges.