PURPOSE: To reduce the difference in the deflecting components of the transmitted light and the reflected light over a visible wavelength region as well as the absorption of light by constituting at least one of the layers of a beam splitter provided with two dielectric thin film layers on both sides with a metal layer in-between by material of ≤1.80 refractive index.

CONSTITUTION: The cube type beam splitter of ng=1.52 sandwiching, for example, Ag of thickness 22nm as a metal film is constituted by vopor depositing a TiO2 film of refractive index n1=2.30 at d1=20nm as the 1st layer, an Al2O3 film of n2=1.68 as the 2nd layer, an Ag film at d3=22nm as the 3rd layer, an Al2O3 film as the 4th layer and a TiO2 film as the 5th layer successively and is made by finally sticking these by an adhesive agent of refractive index ne=1.56. If the reflectance R and transmittance T are measured at 45° in the incident angle from the ng side, the S polarizing component Ts and P polarizing component Tp of the transmitting luminous flux and the respective components Rs, Rp of the reflecting luminous fluxes become such that Ts and Tp; Rs and Rp are the approximate value at the respective wavelengths of visible regions 400W700nm, thus the bright beam splitter of less absorption is obtained.