A waveform of the output signal having high reproducibility to an input signal is outputted from capacitive load. Output signal Vp-n1 appeared at output terminals (50, 51) at two ends of a load L1 is compared with output signals V1a,V1b and input signals Vin fed back to input terminals (9a,9b) to detect errors between signals, a first error inhibiting signal Vout1 is generated for inhibiting the detected errors between the signals, and slope of the first error inhibiting signal Vout1 is detected, a second error inhibiting signal Vout2 is generated for inhibiting slope error between input signals Vin if the slope signal passes the detection, the ratio of time for supplying power to load L1 to that of not supplying power to load L1 is changed according to the error amount of the error inhibiting signal.