PURPOSE: To make the aperture of an eye pattern constant, by providing a carrier wave regeneration and automatic gain control fucntion only by a conventional discriminator and full-wave rectifier so as to obtain an APC signal and an AGC signal without using a multi-value discriminator and a DC multiplier.

CONSTITUTION: After a required gain is provided to an IF input signal, the signal is branched into two, demodulated respectively at phase detectors 2, 3, and four-value base band signals XP, XQ are generated. They are separated into eight regions at binary value discriminators 22, 23, and the levels of I ∼IV are discriminated corresponding to regions 1,2∼7,8, allowing to correspond the level to digital signals DP, DG and to discriminate easily quasi-error signals YP', YQ'. The sum of the signal is used as an AGC control signal, which controls a variable gain circuit 21. Further, the shift in the level of demodulation waveforms XP, XQ is detected respectively as the change in the error signals Yp, YQ, then a difference signal YQDP-YPDQ attains an APC signal corresponding to the phase shift θ.