Phase data 129 of received signal is fed through a one symbol delaying unit 141 and a second substracter 1 to a second decoder 2 for decoding to generate quality data 133c. The quality data 133c is computationally processed to detect frequency deviation. The quality data 133b is obtained by decoding in a first decoder 142 through the one symbol delaying unit 141, a correcting circuit 144 and a first decoder 142. The quality data 133b is used for line control.