The illumination of the side was possible and it was suited for the observation optical system of wide field angle, being small-sized, it offers the illumination optical system where light distribution property is superior. The prism which is laid out on the incident optical axis (2) and the said prism (2) it is laid out in the front step and or the latter part two reflecting interfaces (2b and 2c) the glass lens which it possesses it has (3), the said reflecting interface (2b and 2c) inside at least one the transmitted reflecting interface (2b) the illumination optical system which is offers (1). In addition, the incident optical axis and crossing diagonal, it is laid out, it transmits the portion of the light which incidence is done and begins shooting, deflecting in the direction which the incident optical axis crosses the other at least portion of light, it offers the illumination optical system which has the prism which possesses the surface which it makes begin shooting. In addition, the longitudinal direction inclining vis-a-vis the incident optical axis, it offers the illumination optical system which has the rod lens which is laid out.側方の照明が可能であり広画角の観察光学系に適した、小型で配光特性の優れた照明光学系を提供する。入射光軸上に配置されるプリズム(2)と、該プリズム(2)の前段または後段に配置され2つの反射面(2b,2c)を有するレンズ(3)とを備え、該反射面(2b,2c)のうち少なくとも1つが透過反射面(2b)である照明光学系(1)を提供する。また、入射光軸と斜めに交差して配置され、入射した光の一部を透過して射出し、光の他の少なくとも一部を入射光軸と交差する方向に偏向して射出させる面を有するプリズムを備える照明光学系を提供する。また、その長手方向が入射光軸に対して傾斜して配置されたロッドレンズを備える照明光学系を提供する。
