PURPOSE: To prevent the deterioration of the quality of an RF signal, and to stabilize the CN characteristic by enlarging the transmittivity by using a beam splitter which has provided a dielectric layer whose refractive index is larger than that of a prism between the prisms, and detecting the RF signal.

CONSTITUTION: Proximity surfaces 102, 103 of prisms 100, 101 of a beam splitter 39 make an incident angle of 45° against a luminous flux Is reflected by a beam splitter 34. To this surface 103, a dielectric substance such as TiO2, ZrO2, etc. sticks and forms a single layer film 104 by vapor deposition of an electron beam and it is joined to the prism 100 by an adhesive agent 105. According to such a constitution, the transmittivity to an S polarized light goes to 60W90%, and by detecting an RF signal by a transmitting light side, the quality of the RF signal is improved, and a C/N ratio can be stabilized.