obtaining a phase difference (??n -??n-q) between the detected phase ??n and a phase ??n-q of up to L symbols before, where L is an integer that is 2 or greater; cumulating means for cumulating decided phase differences ??n-1 (q)=??????n-i, where ?? is the sum of the phase differences from i=1 to q-1; metric calculating means for calculating the v-th power value of the absolute value of the diffe