PURPOSE: To simplify a current supplying circuit for supplying current to a hold capacitor by setting the gain of an inclination hold interpolating circuit so that an inclination forecasting circuit is in low frequency, the gain is fixed and its output coincides with the voltage of the hold capacitor.

CONSTITUTION: An input from an input amplifier 2 is supplied to the hold capacitor through a switching circuit 3 and also outputted through an output amplifier 5. Said output is supplied to the inclination forecasting circuit 25 mixing and amplifying signals obtained through the 1st and 2nd differential circuits. The inclination forecasting circuit 25 corrects the voltage and phase of the output from the amplifier 5 by the time constant of the 1st differential circuit so that the output is converted into simple constant current and the simple constant current is made to flow into the hold capacitor 6 through a resistor RL1 ideally. The forecasting of inclination at the instantaneous time starting current flow into the hold capacitor 6 is executed on the basis of the inclination of a signal obtained immediately before the opening of the switching circuit 3.