A video signal detecting circuit includes a synchronization detector for detecting a vertical synchronous signal in an input video signal. A counter starts counting pixel clock pulses in response to every vertical synchronous signal thus detected, and outputs a first signal when the count of pixel clock pulses reaches a preselected value. A comparator compares the vertical synchronous signal detected with the first signal for outputting a second signal representative of a difference between them. A mean circuit produces a mean value of the second signals over a plurality of pictures of the input video signal. An adjusting circuit adjusts the vertical synchronous signal with the mean value to output the resultant adjusted signal as a vertical synchronous signal. The preselected number is substantially equal to the standard number of pixels included in a single picture.