PURPOSE: An actually required wave form is represented as that in a higher standard of precision, by making a high-speed Fourier conversion section longer than that required to be displayed on its wave form, even if distortions are invided on both ends of a reversed high-speed Fourier conversion space after the reversed high-speed conversion process.

CONSTITUTION: Distortions are represented as B. High-speed Fourier conversion FFT section T, and reversed high-speed Fourier conversion iFFT section W start earlier and end earlier than a wave form display section C the wave form of which being required to be represented. Further, aforesaid wave form display section C does not include the distorted portions of the iFFT wave form 4. Consequently, a displayed wave form 5 reproduces the wave form of the original actual signal 2 without distortion. Setting of this particular time is available by setting of a sampling trigger 1. Thus, a wave form can be displayed under less influence of distortions and of high precision.