An apparatus is provided for monitoring a focal state of a microscope having an object plane and a main imaging area. The apparatus has an auxiliary light source providing an auxiliary light beam and coupling the auxiliary light beam into the microscope in such a way that the coupled auxiliary light beam runs within a plane which is spanned outside of the main imaging area by a straight line running in the object plane and a normal to the object plane, and that the coupled auxiliary light beam is inclined at an angle to a normal to the object plane. A part of the coupled auxiliary light beam reflected by a reference boundary surface in the microscope impinges on a registration device in an area of incidence. The registration device registers position changes of the area of incidence on the registration device.