EL222 is a blue light sensor protein, which consists of a light-oxygen-voltage domain as a light sensor and a LuxR-type helix-turn-helix DNA-binding domain. The reaction dynamics of the protein-DNA binding were observed for the first time using the time-resolved transient grating method. The reaction scheme was determined, showing that photoexcited EL222 first binds DNA and the ground state EL222 monomer is subsequently associated with the complex. Rate constants on the millisecond scale were determined for these processes. In addition, binding rates for EL222 with three DNA sequences, with different binding affinities, were measured. Although EL222 binds nonspecific DNA sequences with affinities at least 5-fold lower than the target sequence affinity, the binding rates were almost the same as that for the target DNA. This observation indicates that the specific and nonspecific binding affinities are mainly controlled by differences in the dissociation of DNA binding.