To provide a prism optical system which does not deteriorate the efficiency of color separation due to a color separation prism even when an optical low-pass filter is arranged reversely with respect to front and rear, and to provide an imaging apparatus provided with the prism optical system.

In the prism optical system which is provided with an optical low-pass filter 100 disposed on the incident side of a light beam and a color separation prism 11 of performing color separation of a plurality of colors by an optical thin film 12, the optical low-pass filter 100 is arranged on the incident side of the prism 11, is provided with phase dissolving plates 5, 7, 9 and birefringence plates 6, 8 alternately arranged and is constituted so as to satisfy |X1/X2|>0.7, wherein the efficiency of phase dissolution at 550 nm when the phase dissolving plate 5 of the incident side of the light beam is entered between crossed nicols is X1 and the efficiency of phase dissolution at 550 nm when the phase dissolving plate 9 of the emission side is entered between crossed nicols is X2.