To provide a dynamic focus circuit which has a simple circuit configuration and is inexpensive, and is capable of providing optimal performance over the entire area of a screen of a cathode ray tube.

The dynamic focus circuit 1 is provided with a parabolic waveform forming part 2, and a waveform converter 3 for converting the waveform of a parabolic waveform voltage 5. The converter 3 is provided with an amplification factor adjustment 12a, 12b whose voltage-current characteristic changes in response to an applied voltage, and a threshold adjusting voltage supply 20 for supplying a threshold adjusting voltage for adjusting the threshold of the adjustments 12a, 12b. The converter 3 amplifies the inputted parabolic voltage 5 by an amplification factor according to the voltage-current characteristic of the adjustments 12a, 12b.