Disclosed is an AGC detector device and an AGC detecting method for television receivers displaying video pictures consisting of a plurality of horizontal lines to be built up successively, wherein a CVBS signal is inputted which includes horizontal sync pulses having a front porch region and a back porch region and occurring once a horizontal line during a horizontal sync period when generating a current video picture, and further includes vertical sync pulses occurring during a vertical sync period before the generation of a new video picture and including serration pulses which occur during a serration pulse region being part of the vertical sync period. Further, gating pulses are generated having a period which is equal to the line period of the horizontal sync pulses. Said gating pulses are adjusted such that they occur at the back porch region of the horizontal sync pulses. The particularity of the invention is that the occurrence of the gating pulses is shifted during the serration pulse region of the vertical sync period so that the gating pulses occur earlier by a time interval corresponding to the time distance between a front porch region and the next back porch region or later by a time interval corresponding to the time distance between a back porch region and the next front porch region.