Abstract: In this invention, a reading signal detected by a photodetector is input into two differentiating circuits of different differential constants. The first differentiating circuit differentiates the reading signal with the differential constant larger than that of, for example, the second differentiating circuit and lowers the level of the noise component present as mixed in the reading signal. The first comparing circuit removes the noise component, for example, below the threshold value. The second differentiating circuit differentiates the reading signal with the differential constant converting the peak point of the reading signal, for example, of the above mentioned photodetector to a zero crossing point. The second comparing circuit compares the differential signal of the above mentioned differentiating circuit with the threshold value set, for example, on the zero level and makes the output signal rise, for example, in the center position of an information pit.