Abstract: An illuminating device for an operation microscope having a light source including a light guide with a light-exit surface centered on an illumination optical axis. An optical system is arranged on this axis between the light-exit surface and the main objective of the operation microscope, and the optical system comprises a converging lens, an aspheric lens and a variable-aperture diaphragm. This optical system is displaceable in the direction of its axis, and this displacement is coordinated with aperture setting of the diaphragm. The light-exit surface of the light guide is so positioned with respect to the optical system and with respect to the main objective as to illuminate the operation field with requisite size and light-distribution.