An object of the present invention 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 1 includes a photoconductive switch 18 which receives predetermined pulse light from a first laser light source 11, and outputs terahertz light L1 having the same repetition frequency frep1 as the repetition frequency of the predetermined pulse light, a photoconductive switch 21 which receives the terahertz light L1 and a sampling light pulse L2, and outputs signal corresponding to a power of the terahertz light L1 at a time point when the sampling light pulse L2 is received, an RF spectrum analyzer 26 which measures a magnitude of the signal corresponding to a measured frequency which changes over time, an optical coupler 32 which outputs a simultaneous light pulse when the predetermined pulse light and the sampling light pulse L2 are simultaneously input, a photo detector 36 which converts the simultaneous light pulse into an electric signal as a trigger signal EXT, and an optical delay circuit 34 which delays the trigger signal EXT.