Extrapolation judging equipments (1441, 1442) which deal with the head and the end of a data line notify filters for analyzation of returning modes (P, Q) sent from symmetry judging equipments (1431, 1432) and decide appropriate returning modes (P', Q') based on phase shift amounts (dP, dQ) sent from the symmetry judging equipments (1431, 1432) and send the decided returning mode (P', Q') to synthetic filters. In the case that the filters for analyzation are even functions, a zero shift even function returning mode is notified if the absolute value of the phase shift amount is zero and a half shift even function returning mode is notified to the synthetic filters as extrapolation for the synthetic filters if the absolute value of the phase shift amount is D/2. In the case that the filters for analyzation are odd functions, a zero shift odd function returning mode is notified if the absolute value of the phase shift amount is zero, and a half shift odd function returning mode is notified to the synthetic filters as extrapolation for the synthetic filters if the absolute value of the phase shift amount is D/2.