An object is to enable a change in a frequency for which an electric signal based on an optical signal is measured by a spectrum analyzer. An optical measurement device includes a first photoconductive switch that receives predetermined pulse light from a first laser light source, and outputs terahertz light having the same repetition frequency as the repetition frequency of the predetermined pulse light. The optical measurement device also includes a second photoconductive switch that receives the terahertz light and a sampling light pulse, and outputs a signal corresponding to a power of the terahertz light at a time point when the sampling light pulse is received. The optical measurement device further includes an RF spectrum analyzer that measures a magnitude of the signal corresponding to a measured frequency that changes over time, an optical coupler that outputs a simultaneous light pulse when the predetermined pulse light and the sampling light pulse are simultaneously input, a photo detector that converts the simultaneous light pulse into an electric signal as a trigger signal, and an optical delay circuit that delays the trigger signal.