change in the phase comparing voltage a from 0 5 v to 0 5 v is detected by the differentiation circuit 15 which provide a differentiator output signal as shown in fig3 b a waveform shaping of the differentiator output signal is in turn effected in the window comparator 16 to provide a pulse signal shown in fig3 c thus the detection of the inversion of the burst phase is performed the operation of the circuit part including the phase comparator 19 is performed in the same manner during this period as illustrated in fig2 the operation is expressed by a movement from a point a which is coincident with the point a to a point a 39 which is coincident with the point a 39 this operation of the circuit part including the phase comparator 19 is also illustrated in fig3 d through 3f under this condition the detection of the inversion of the phase is detected by both of the circuit part including the phase comparator 13 and the circuit part including the phase comparator 19 thus the inversion detection signal is derived at an output terminal of the or gate 17 in addition the polarity of the differentiator output signal also becomes negative when the change in phase occurs in the reverse direction i e from the point a 39 a.