1. A control circuit of a rear-vision mirror including a liquid crystal mirror for use in automobiles, for automatically operating the liquid crystal mirror which reduces incident light during application of a driving voltage to the liquid crystal mirror, the circuit comprising: light sensing and comparing means for sensing incident light on a liquid crystal mirror, providing a sensor signal representing the amount of incident light, comparing the sensor signal with a predetermined reference signal to determine strength and weakness of the incident light, and providing a comparison signal indicating the amount of the incident light; an Oscillator for receiving the comparison signal from said light sensing and comparing means and generating a square wave having a constant period; pulse generating means for receiving the square wave from said oscillator and generating a pair of operating voltage driving pulses having amplitudes determined by a source voltage and having different phases; and voltage driver means for buffering the driving pulses of said pulse generating means and providing the driving pulses to said liquid crystal rear-vision mirror.