PURPOSE: To improve the output duty of a pulse amplitude modulated wave of an A/D converting system and to improve the quality of the system by outputting interpolatively an average value of an analog signal output corresponding to an input digital signal and a signal output corresponding to the just preceding input signal.

CONSTITUTION: The digital code signal from an input terminal 1 is inutted to a D/A converter 2 of a D/A converting system, and the output of this converter 2 is applied to one terminals of the first and the second switches 3 and 9. A capacity 4 is connected between the other terminal 13 of the switch 3 and a reference power source 5, and the switch 9 is connected to one terminal of the third switch 11, and a capacity 10 is connected between the reference power source 5 and the connection point between switches 9 and 11, and the other output of the switch 11 and the output of the switch 3 are inputted to a buffer amplifier 6. Switches 3, 9 and 11 are controlled by switch control signal 1, 2, and 3 from a switch control signal generating circuit 8 respectively, and an average value of the output corresponding to the input and the output corresponding to the just preceding input is outputted interpolatively, thus improving the output duty.