PURPOSE: To reduce the effect of polarization by forming a light semitransmitting film composed of three layers of the first dielectric layer, a silver metal layer, and the second dielectric layer between prisms.

CONSTITUTION: A light semitransmitting film HM is formed between the junction faces of two right triangular prisms 2, 4 of n0 refractive index. This film HM is composed of the three layers of the first dielectric layer M having a refractive index of n1 and an optical thickness of n1d1, a silver metal layer, and the second dielectric layer having n2 and n2d2. C in the figure is an adhesive layer. The following relationships are satisfied: n0<n1, 1.55≤n1≤1.75, 1.85≤n2≤2.45, n1d1=n2d2=0.25λ0, where λ0 is a designed wavelength selected within the range of 400W700nm, and the algebraic thickness of the silver layer is selected within the range of 10W50nm in accordance with a desired reflectance to transmittance ratio.