PURPOSE: To eliminate effectively the AC noise for an analog input signal fetching device which performs an A/D conversion, by using a clock pulse signal which sets the time of integraion of an integral A/D converter at an integer- fold value of the power supply frequency and at the same time setting the cycle of the AC signal used to an input circuit at a fraction of an integer of the time of integration.

CONSTITUTION: The analog input signal supplied to an input terminal 1 is fetched by an input circuit 2 and integrated for a prescribed time by an integral A/D converter 4 which is insulated by an isolator 3 to be converted into a digital signal. This digital signal is delivered through a terminal 5. The frequency of a power supply AC which actuates each part of the circuits 2∼4 is detected by a detector 6. A frequency divider 81 produces a clock pulse signal so as to set always the time of integration of the converter 4 at an integer-fold value. The output pulse of the divider 81 is divided furthermore, and a driving signal of a fraction of an integer of the integration time of the converter 4 is supplied to the circuit 2.