PURPOSE: To attain an AD converter in which noise in commercial power supply is eliminated and which has a wide dynamic range, by integrating input signals only for a time being an integer multiple of the period of a commercial power supply frequency an AD-converting its integrated voltage.

CONSTITUTION: In selecting a switch 26 to the S1 position and opening a switch 23 only with switches 23, 25 closed, an integrated voltage E1 of a variable gain integrator 22 becomes as shown by Equation 1 after a time (t) when digital codes given to a multiplying AD converter MDAC3 are D0∼Dm. In taking (t) as an integer multiple of the period of the main noise (commercial frequency noise) included in an input Ex, the noise is eliminated with the integrarion effect. A μP5 activates a smaple hold circuit 28 in a prescribed time satisfying the condition and holds the integrating voltage E1 for the time, and in selecting the switch 26 to the S2 position, a reference voltage Es is applied to the MDAC 3 to perform the AD conversion with the comparison system.