PURPOSE: To ensure the action switching stability by setting the action detecting points in the vertical blanking periods of both odd and even fields and securing a hysteresis operation with two window signals.

CONSTITUTION: A detection signal OD is produced at an odd field operating range detecting part 3 based on the phase relations among the window signals OW1 and OW2 and a reference pulse signal OR. Meanwhile the window signals EW1 and EW2 and a reference pulse ER are inputted to an even field operating range detecting part 4, and a detection signal ED is produced. Both signals OD and OE are inputted to an action deciding part 5, and a detection signal OD-1 and a detection signal ED-1 are held in the present odd and even fields respectively. It is checked whether the different states are confirmed or not between the signal OD-1 and the next signal OD to be inputted and between the signal ED-1 and the next signal ED to be inputted respectively. Then a switch control signal CNT is outputted through an output terminal OUT to ensure the switch to a 1-horizontal scanning period delaying action according to the different states of the preceding signals.