To realize a polarizing beam splitter and an optical isolator optical system in which a displacement from an optical axis is suppressed within a minimal extent even when an error or a variation exists in a fitted angle.

The polarizing beam splitter is characterized by the fact that the beam splitter is provided with a pentagonal prism 10 in which a light beam which is made incident on a first surface 11 is emitted from a second surface 12 as an emitted beam which has a declination of 90° or 270°, a polarizing film 30 which is furnished on a fourth face 14, reflects a light polarized in a first polarizing direction and passes a light which is polarized in a second polarizing direction perpendicular to the first polarizing direction, a prism body 20 which is in contact with the polarizing film and has an incident face 21 perpendicular to the first face. A function specific to a pentagonal prism is effectively utilized, a light beam is emitted as one which is in parallel to an optical axis which is slightly displaced from the optical axis even when the fitting position of the beam splitter is slightly turned. An incidence on the same position on a body or a light detector as that which is given by a normal fixed state is possible by the existence of a lens system.