An operation microscope includes a microscope body having an illumination optical system which illuminates a subject, a lens barrel, and a main observation optical system which observes the subject, an assistant's microscope unit attached to the microscope body, and a guide rail which is disposed in the lens barrel of the microscope body, and extends in a circumferential direction about a center of an optical axis of an objective lens of the microscope body. The assistant's microscope unit is disposed in the guide rail to be movable between a usage position and a non-usage position in a circumferential direction of the lens barrel.