Abstract: An apparatus for detecting an information recorded area on an information recording medium having the information recorded area and an information non-recorded area. The apparatus includes an optical head for radiating a light beam onto the information recording medium, a motor for rotating the information recording medium. In the apparatus, an electrical signal corresponding to a reflected light reflected by the information recording medium while the information recording medium is rotated is generated. The electrical signal corresponds to the information recorded in the information recorded area, and the electrical signal changes between a first potential level and a second potential level which is different from the first potential level. Further, in the apparatus, a threshold signal is generated by dropping the first potential level with a prescribed potential and raising the second potential level with the prescribed potential, and delaying the electrical signal.