FIGS. 5B and 5C respectively shoW the inputs and the outputs of the ramp signal generating circuits 3 and 11. The output of the ramp signal generating circuit 3 is dropped to the ground level, for example, When the pulse from the oscillator 2 is input. The output rises at the speed determined according to the current value generated by the constant current source and the capacity of the capacitor Cx, until the next pulse is input. Thereafter, the ramp signal generating circuit 3 repeatedly generates and outputs this Waveform.