A time-domain waveform of a terahertz wave is measured by a method based on time-domain spectroscopy by using an optical delay unit to adjust an optical path length along which excitation light propagates thereby adjusting a difference between a time at which the excitation light arrives at a generating unit configured to generate the terahertz wave and a time at which the excitation light arrives at a detection unit configured to detect the terahertz wave. The optical delay unit is driven according to a first speed pattern to acquire a first time-domain waveform. The optical delay unit is then driven according to a second speed pattern different from the first speed pattern to acquire a second time-domain waveform. The first time-domain waveform and the second time-domain waveform are averaged.