To speedily converge the amplitude of a vertical deflection waveform to a preset amplitude control target in a transient state just after switching a vertical deflection frequency at the time of changing a picture display size and to improve the stability of the amplitude control system in a steady state after switching.

This device is provided with a waveform output circuit 21 for outputting the deflecting waveform at the vertical deflection frequency by working an arbitrary sample waveform and an amplitude control circuit 22 for controlling the amplitude of the vertical deflection waveform to be constant based on the input difference between the vertical deflecting waveform outputted from this waveform output circuit 21 and a preset amplitude control signal S2. When the input difference between the amplitude control signal S2 and a signal Sv based on the vertical deflection waveform is a little, the amplification factor of an error amplifier 23 is set low but when the input difference between the amplitude control signal S2 and the signal Sv based on the vertical deflection waveform is large, the amplification factor is set high.