In an illumination optical system for a DMD type projector, the TIR prism is configured such that a projection line, which is a normal to the total reflection surface vertically projected on the DMD surface, forms an angle other than 90° with a micromirror turning axis, an exit light projection line, which is a principal ray of light beam exits from the total reflection surface vertically projected on the DMD surface, forms an angle of 90° with the micromirror turning axis, and an incident light projection line, which is a principal ray of light beam incident on the total reflection surface vertically projected on the DMD surface, is located on the same side as the exit light projection line with respect to a straight line passing through an end point of the incident light projection line and extending parallel to a long side of the DMD.