PURPOSE: To provide the method for preparing automatic gain control data in a signal processor, etc., for processing various kinds of signals.

CONSTITUTION: A threshold level is obtained 5 from the total value of all sampling data in an arithmetic block written in a storage area, the difference between the sampling data exceeding this threshold level and an initially set replacement (fixed value) is obtained and defined as a corrected value to obtain 8 the total value of those corrected values, and the corrected sampling data are obtained by converting the sampling data exceeding the threshold level into fixed value by subtracting 9 the total value of corrected values from the original total value of all the sampling data. The total value of these corrected sampling data is obtained 10, a mean value is obtained 11 by dividing this fixed value with the number of sampling, and this average value is defined as automatic gain control data. Thus, high-speed processing is enabled without damaging the original accuracy of automatic gain control or reloading the contents in the storage area M.