The recently developed near-field heterodyne transient grating method was utilized in the analysis of photochemical reaction dynamics. It was applied to the dynamics measurement of the dimerization reaction of anthracene in the temporal range from nanoseconds to milliseconds. By means of wide dynamic range measurements, the relaxation processes of excited states such as the excimer and triplet, and the diffusion process of the photodimer product, could be directly observed. Relative photodimerization efficiency obtained experimentally was compared with the simulation results of the reaction kinetics on the basis of two different reaction schemes—reaction by way of the singlet or triplet excited states—and it was confirmed that this reaction occurs through the singlet excited state.