PURPOSE: To stabilize color synchronization, by comparing a correction signal, synchronizing with a vertical retrace signal, with a trapezoidal signal, synchronizing with a horizontal retrace signal, and by gating a line-in signal following up the timing of a line-in signal detected by a picture tube with the comparison output.

CONSTITUTION: Vertical synchronizing correction signal generator 34 generates a correction signal synchronizing with a vertical retrace signal, and horizontal synchronizing trapezoidal wave generator 32 generates a trapezoidal signal synchronizing with a horizontal retrace signal; and then, both the signals are supplied to comparator circuit 33, which generates an output with pulse width equivalent to a comparative voltage. Following up the timing of a line-in signal detected by index-type color picture tube 21 with the output of comparator 33, the line-in signal is gated 27 accurately, thereby stabilizing color synchronization.