Abstract: An automatic focus adjusting system for a surface inspection microscope including a sheet setting stage, a microscope for observing a sheet surface set on the stage, a television camera for picking up an image observed by the microscope and a focus adjusting system. The system irradiates the back or front of an optically transparent sheet set on the stage creating an image on the front surface of the sheet. The image's contrast varies depending on the inner physical structure of the sheet. The television camera's video signals are processed by a focus evaluation circuit. The image contrast of the sheet's front surface image generates a contrast frequency allowing the focus evaluation circuit to calculate the focus evaluation degree. The system automatically adjusts the stage and compares the present focus evaluation degree with the prior focus evaluation degree.