PURPOSE: To improve the amplitude characteristic and the group delay characteristic by applying an output of a high speed A-D converter to a digital filter sampling a signal by using the 1st sampling frequency so as to obtain a prescribed digital signal.

CONSTITUTION: The A-D converter 10 consists of a low pass filter 11, the A-D converter 12 of high speed operation and the digital filter 13. The cut-off characteristic of the amplitude characteristic in using the digital filter 13 is brought into a characteristic steeper than that of a low pass filter 11 as shown in Curve (e) by selecting properly the weighting, and then the overall amplitude characteristic is as shown in Curve (f) so as to obtain the cut-off characteristic where the characteristic is flat in the pass band and steep in the cut-off band comparatively. Similarly, since the group delay characteristic of the digital filter 13 is a characteristic, at least flat in the pass band as shown in Curve (g), the overall group delay characteristic of the A-D converting system is a characteristic almost flat over the entire pass band as shown in Curve (h).