PROBLEM TO BE SOLVED: To provide a polarization separation element, optical system and optical instrument that deal with a wider angel of incidence due to a simple multilayer film (a laminate film) without requiring a structure birefringent film.SOLUTION: A polarization separation element has a multilayer film composed by having a relatively low-refractive index substance and high-refractive index substance alternately laminated between two pieces of first base material and second base material. A pair of two layers of a low-refractive index substance layer and a high-refractive index substance layer adjacent to the low-refractive index substance layer is denoted as one stack. In order from the first base material, when an n-th stack is denoted as the n-th stack, an optical film thickness of the low-refractive index substance of the n-th stack is denoted as Ln, an optical film thickness of the high-refractive index substance of the n-th stack is denoted as Hn, the polarization separation element is characterized in that the stack more than a half of the multilayer film satisfies a following conditional expression (1). H1:L1=Hn:Ln (1)SELECTED DRAWING: Figure 1【課題】構造複屈折層を必要とせず、単純な多層膜（積層膜）により幅広い入射角度に対応する偏波分離素子、光学系及び光学機器を提供すること。【解決手段】２枚の第１基材と第２基材の間に、相対的に低屈折率物質と高屈折率物質を交互に積層して成る多層膜を有する偏波分離素子であって、 低屈折率物質層と低屈折率物質層に隣接する高屈折率物質層の２層をペアで１スタックとし、第１基材より順に、ｎ番目のスタックを第ｎスタックとし、第ｎスタックの低屈折率物質の光学膜厚をＬｎ、第ｎスタックの高屈折率物質の光学膜厚をＨｎとした場合、多層膜の半分以上のスタックが、以下の条件式（１）を満足することを特徴とする偏波分離素子である。 Ｈ１：Ｌ１＝Ｈｎ：Ｌｎ （１）【選択図】図１
