Abstract: An operation microscope system comprising an operation microscope including an observation optical system adapted for observing a portion of a patient wherein an operation is to be made. A mark device is provided on one of the microscope and the patient's portion and a mark position detector is provided on the other.A control circuit is provided and has a memory for memorizing an initial position signal as produced by the detector when an initial focus condition or a visual field is established between the patient's portion and the microscope and an out-of-focus condition or a change in the visual field is detected by a relative displacement between the patient's portion and the microscope based on the initial position signal and an actual position signal as produced by the detector to produce a correcting signal to thereby restore the initial focus condition or the initial visual field.