Provided is an operation microscope apparatus for easily and speedily obtaining a suitable separation state between illumination light and observation light in a case where a front lens is replaced by another one. An illumination optical system includes a slit plate in which slit holes having different slit widths are formed. Each of a plurality of front lens having different refractive powers is integrally formed with corresponding one of storing sections. One of the storing sections is attached to an elevation regulating member. The elevation regulating member includes micro-switches. Each of the storing sections includes a protruding portion for turning on corresponding one of the micro-switches at the time of attachment. A control unit sets an illumination angle of the illumination light and a slit width of a slit based on a detection signal from a micro-switch which becomes an on state.