PURPOSE: To obtain an optimum focus, through the use of the sampling of the level of high frequency component at the point of time delayed by H from the zero crossing point, by picking up the high frequency component of video signals and detecting the zero crossing point of the high frequency component.

CONSTITUTION: A video signal E18 is inputted to an HPF40 and a high frequency component E40 is picked up. The high frequency component E40 is inputted to a zero crossing sensor 42 and converted into the 1st signal E42. The E42 is delayed at a phase delay circuit 44 to be the 2nd signal E44. The 2nd signal E44 is inputted to a sampling pulse generator 46. The generator 46 is triggered at the leading of the 2nd signal E44 to generate the sampling pulse E46. The E46 is given to a sample hold circuit 48. The signal level of the high frequency component E40 sample-held at the circuit 48 is outputted as the 3rd signal E48. The E48 is inputted to a peak detector 50, which outputs a signal corresponding to the peak value of the E48. The E50 is amplified at the amplifier 52 to be the signal E52 having a suitable level, and the potential of the E52 becomes a signal representing the degree of the focus adjustment.