PURPOSE: To reduce the cost of a hardware by periodically inputting impulses to a system to be measured, taking an arithmetic mean with the moment when the maximum value of a response waveform is obtained as a synchronous point and obtaining the averaged response waveform.

CONSTITUTION: An impulse generated from an impulse generator 11 is inputted to a system 12 to be measured and the system 12 outputs impulse response. This impulse response is collected by a microphone 13 and recorded by a data recorder 14. The impulse response is further taken in by a computer system 15 and stored in a memory 17 in the system 15. Then, a time point of Δt temporally before the moment when the maximum value of the impulse response is obtained is set as a starting point and the arithmetic mean of data in a section with a mean impulse interval T is taken from the data stored in the memory 17. That is, the maximum value is retrieved such that a local maximum value retrieving window with the mean impulse interval T is determined and, when the window is larger than a previous maximum value candidate, the window is made the present maximum value candidate. When the present maximum value candidate is not updated over the section of 2/3T, the present maximum value candidate is made the true maximum value. Thus, the cost of a hardware can be reduced.