PURPOSE: To compress larger amount of information when information amount with a periodic signal is compressed.

CONSTITUTION: A floating storage means 2 successively writing the amplitude value of a specified section on input signal waveform while shifting addresses and measuring the time width of the specified section, an amplitude most approximate point decision means 3 outputting the amplitude difference value for each address showing the least amplitude difference total value after divided with the lowest difference value output command number after obtaining m- number of amplitude difference total values which are the difference of the amplitude value of the inputted basic waveform and the amplitude difference value which is the difference of the amplitude value of the same address from the drift storage means against n-number of addresses other than the leading address, a time width most approximate point decision means 4 deciding the most approximate point of the time width, and a basic waveform forming means 1 forming the average basic waveform by successively adding the amplitude value and the time width storing the amplitude difference value and the time width difference value for each address, are provided.