A photoreaction of caged calcium (Ca 2+ ) was investigated by using the time-resolved transient grating (TG) method. The q 2 -dependent feature of the TG signal was analyzed based on a model that there are two parallel dissociation steps with different rates and Ca 2+ is released promptly after the fracture of the caged compound. The TG signal representing the presence of Ca 2+ was appeared by the volume contribution. Although the diffusion coefficients of Cg and the decomposed product are different without Ca 2+ , only one diffusion component was observed after the dissociation of CgCa 2+ , indicating that the caged compounds and Ca 2+ diffuse together.