In an A/D conversion apparatus comprising an A/D converter (4) and a digital moving average filter (6) for removing high-frequency signal components, there is provided an analog moving average filter (2) at a first stage. A sampling frequency of the filters (6 and 2) is set to fsd=n.times.fsa (where n is a positive integer of 1, 2, ---). As a result, it becomes possible to superimpose an unnecessary signal passing area that appears at every frequency of n times the sampling frequency fsd in the filter (6), on an area of infinite attenuation that appears at every frequency of n times the sampling frequency fsa in the filter (2). In the apparatus as a whole, it becomes possible to efficiently attenuate unnecessary high-frequency signal components. It is also possible to obtain a similar effect when a time A/D converter is used in place of the analog moving average filter (2) and the A/D converter (4).