PROBLEM TO BE SOLVED: To surely and easily display a desired change point in a signal waveform.SOLUTION: When a first operation on a first portion on a waveform display screen and a second operation on a second portion different from the first portion are performed in this order in a state where a start condition is satisfied, a first signal level corresponding to the first portion and a second signal level corresponding to the second portion are specified respectively (step 22), and the first signal level is taken as a designated signal level to specify a first change point (steps 27a and 27c) if the second signal level is higher than the first signal level (step 26), and the first signal level is taken as a designated signal level to specify a second change point (steps 26, 27b, and 27d) if the second signal level is lower than the first signal level (step 26), and a signal waveform within a preliminarily defined time range including the specified change points is displayed on the waveform display screen (s
