To accumulate input digital logic signal information more than memory capacity without losing important information, by compressing logic samples in real time before accumulating them in a capturing memory in capturing the logic samples.

A capturing circuit 105 samples input digital logic signals and generates logic samples. A compression circuit 140 accumulates the logic samples into a compression code, and accumulates them in the capturing memory 115. A wave form drawing circuit 120 converts the compression code into a waveform image, accumulates them in a display memory 125, and displays them on a display device 130.