1. An automatic image control device comprising: a lower-sideband variable-peaking circuit which receives an amplified FM signal that is output from a video head and that has had its frequency spectrum shifted to prevent the occurrence of black-and-white reversal phenomenon, wherein the peaking circuit peaks the lower sideband of the FM signal while maintaining an amplitude ratio between the lower-sideband and carrier wave of the FM signal constant; a lower-sideband filter for extracting the lower-sideband spectral component from the FM signal subjected to peaking; a comparator for comparing a d.c. voltage corresponding to a maximum detected amplitude of the extracted lower-sideband component with a reference voltage to generate an error voltage, wherein the error voltage is fed back to the lower-sideband variable-peaking circuit to regulate jutting amplitudes in the frequency spectrum of the lower-sideband to a uniform value, thereby improving sharpness of an image; and an envelope detecting circuit for detecting the envelope of the voltage of the FM signal output from the video head, wherein the d.c. voltage corresponding to a maximum detected amplitude of the extracted lower-sideband component is overlain by the envelope-detected voltage.