An illumination device for a light microscope with an objective, in particular for a stereo-microscope, illuminates an object plane of the light microscope. An illumination beam path of the illumination device is defined by a light source, a field diaphragm, illumination optics with at least one illumination lens, and at least one deflection element. The object plane of the microscope is obliquely illuminated with incident illumination, wherein an axis of the illumination beam path forms an angle β greater than 0° with an optical axis of the objective. The field diaphragm, the illumination optics and the deflection element are arranged and orientated relative to one another such that a diaphragm plane which is defined by the field diaphragm, an illumination lens plane defined by the illumination lens, and an image plane of the image of the field diaphragm, which is produced by the illumination optics, without deflection by the deflection element, intersect at least approximately along a common straight line and that the image plane of the image of the field diaphragm which is produced by the illumination optics and which is deflected by the deflection element, runs in or parallel to the object plane. Sharp, undistorted imaging of the field diaphragm into the object plane or into a plane running parallel thereto is achieved.