PURPOSE: To keep the screen of a CRT at appropriate brightness and to prevent a power from being consumed wastefully due to excessive luminance by varying the blanking level of a video amplifier while matching with the synchronous frequency of a synchronous signal.

CONSTITUTION: The blanking level consisting of a frequency/voltage conversion circuit 49 and an operational amplifier 29 can be varied corresponding to the synchronous frequency of a horizontal synchronizing signal. The frequency/ voltage conversion circuit 49 is operated so that the higher the frequency of input, the more the voltage of output is boosted. For example, the voltage of a point A is varied corresponding to the synchronous frequency of the horizontal synchronizing signal. Therefore, the voltage of the point A is dropped as the frequency of the synchronous signal inputted to a display device is increased in the circuit and a prescribed shape video signal is supplied to the cathode 43 of the CRT which brightens the screen. Also, the voltage of the point A is boosted as the frequency of the synchronous signal is decreased and the prescribed shape video signal is supplied to the cathode 43 of the CRT which suppresses the luminance of the screen of the CRT as a whole.