signal as described above according to the first embodiment of the invention operation defects can quickly be detected and an accurate servo operation can be maintained without being affected by defects accordingly this embodiment is particularly advantageous in cases when the optical disk is rotated at a high speed 1 800 r p m and a wide servo band width of kilohertz is employed since the servo system is little affected by internal disturbance such as when the servo band is broadened a second embodiment of the present invention will next be described with reference to fig1 and 11 fig1 is a block diagram showing a tracking circuit two rf signals corresponding to the wobbled pits in the respective servo byte region are applied to a terminal 81 and compared in a comparison circuit 90 to produce a signal representing the difference between the level of these two signals the difference signal is input to a first latch circuit 71 latched in response to clock pulses supplied from a terminal 82 and then supplied to a comparator circuit 74 a second latch circuit 75 latches the previous sampling value a value l 1 at a time instant t 1 in fig1 a window generating circuit 72 adds or subtracts a window data of a predetermined width u00b1 w 1 supplied from a terminal 83 to or from the latched value l 1 to thereby produce a window value having a predetermined width l 1 u00b1 w 1 a comparator 74 compares this window value with the current sampling value a value l 2 at a time instant t 2 in fig1 latched in the first latch circuit 71 and.