There has been a circuit in which when a phase compensation change amount exceeds a threshold, occurrence of a slip is detected, and, in this case, a phase correction amount calculated from the phase compensation change amount is added to the phase of a signal for which phase deviation is compensated for, so that a phase change caused by the slip is corrected. For example, the circuit is disclosed in International Publication Pamphlet No. WO 2012/132103. There has also been a method in which SYNC bursts are periodically inserted in an optical signal, known signals such as knows bits or codes are embedded between the successive SYNC bursts, and detection of a cycle slip and correction are performed using the known signals on the receiving side. For example, the method is disclosed in International Publication Pamphlet No. WO 2010/063092.