A sweep termination generating circuit adopts an octagon boundary for sweep termination which is set by combining a plurality of comparators so as to be circumscribed about the display screen of a circular CRT. When the sweep position is deviated from the center of the display screen for the purpose of displaying a radar image by PPI sweep, the circuit allows the comparators to detect that a scanning signal for scanning an electron beam from the off-center sweep position has reached the boundary for sweep termination and thereby generates a sweep termination signal.