Optical scanning apparatus with focussing system

An apparatus for optically scanning a record carrier on which information is recorded, which apparatus is provided with a radiation source for emitting a scanning beam, a read detector for detecting the information contained in the scanning beam after cooperation with the record carrier and a focussing system for focussing the scanning beam on the record carrier, which focussing system comprises a focussing detector for supplying a focussing measuring signal which is representative of the extent to which the scanning beam is out of focus on the record carrier and a focussing control device for controlling the focussing of said scanning beam depending on the measuring signal, characterized in that the apparatus is furthermore provided with an amplitude detector, which is coupled to the read detector, for supplying an output signal which represents the amplitude of the information signal read, a phase detector having a first input for receiving a focussing error signal, a second input for receiving the output signal of the amplitude detector, and an output for supplying a correction signal having a polarity which depends on the phase relationship between the signals applied to the two inputs, and a difference stage for subtracting the measuring signal supplied by the focussing detector and the correction signal supplied by the phase detector from each other and applying the difference signal to the focussing control device.