The method of the present invention is capable of securely detecting LPP signals. The method comprises the steps of: reading a push-pull signal including a wobble component and LPP components; and slice-shaping the push-pull signal with a slice signal so as to binarize the LPP components and detect the LPP signals. The method is characterized by: detecting pulse widths of the LPP signals with changing voltage of the slice signal; calculating degree of dispersion of the pulse widths of the LPP signals; stopping the change of the voltage of the slice signal when the degree of dispersion exceeds a predetermined threshold value; and correcting the voltage of the slice signal on the basis of a predetermined correction value.