To uniformly illuminate a wide irradiation surface by one optical element having the small number of constitution even through a whole device is compact and light in weight.

This illumination optical system is for making an illumination surface 4 illuminate light radiated from a light source 1 and provided with a prism 3 having an action for condensing the divergent light radiated from the light source 1, consisting of three surfaces and consisting of a medium of which the refractive index is larger than 1. Then, luminous flux emitted from the light source 1 is made incident on the inside of the prism 3 from the incident surface 11. Further, the incident light is transmitted through the inside of the prism 3 and reflected from the surface 12 having a transmitting action and a reflecting action. Then, the reflected light is transmitted through the inside of the prism 3 and reflected from a reflection surface 13. Moreover, the reflected light is finally emitted outside the prism 3 from the surface 12 having the transmitting action and the reflecting action. Thus, since an optical path formed in the prism 3 is folded, the illumination optical system is made compact.