PURPOSE: To execute the GAIN control of a video amplifier, a cut-off adjustment, and a white balance adjustment by generating a synchronizing pulse for clamping and a white pattern signal which are synchronous to each other using the low pressure winding wire output pulse of a fly-back transformer, and switching them with a pattern changeover switching means.

CONSTITUTION: Means 60, 62 for generating the video signals which express the timing pulse for clamping and the white pattern and are synchronous with each other, a pattern changeover switching means 68 which switches a CRT screen with a white pattern mode or a black pattern mode, and a means 66 which detects whether or not an external video signal is inputted to a video input terminal and prohibits the use of the synchronizing pulse for clamping and the white pattern video signal when it is inputted, are provided. Therefore, the low pressure winding wire output pulse of a fly-back transformer 50 is generated in a horizontal fly-back period, so the waveform of this pulse can be formed. Thus, the GAIN control of the video amplification circuit, the cut-off adjustment, and the white balance adjustment can be executed without connecting a special external video signal generator.