The mode-locking frequencies of two femtosecond laser light sources are controlled so that they are stabilized at high degree and the difference between the mode-locking frequencies is constant. The output of the laser light sources are used as a pumping light for generating terahertz pulses and a probe pulse light for terahertz detection. Since the time delay timings of the terahertz pulses and the probe pulse light the pulse periods of which are slightly different from each other shift from each other and the difference increases. Therefore, the temporally expanded terahertz pulses are measured by high-speed sampling without using any mechanical stage for time delay scanning. The terahertz electric field time waveform measured by high-speed sampling with the measurement time window of the pulse period is subjected to time-scale conversion and Fourier transform.