The difference signal indicating a difference between the output of the horizontal deflection circuit unit and a desired horizontal raster signal is converted into a digital signal corresponding to the difference signal by the A/D converter. The converted digital signal is outputted to the digital signal processor. The digital signal processor performs a feedback control of raster width by digital operations. The digital signal processor outputs the result signal of the operation to the D/A converter where the result signal is converted into an analog result signal. The analog signal is sent to the horizontal deflection circuit unit where the power to be supplied to the deflection yoke is controlled.