Since conformation changes of proteins and biomolecular interactions (including protein-DNA, or protein-protein interactions) are essential processes for biological functions, detections of these processes are important in chemistry and biochemistry to understand the reactions. For the detection of these processes, a variety of techniques have been developed. UV/vis absorption spectroscopy or emission spectroscopy are very powerful to trace the time development of reactions. However, these techniques have a limitation to detect the conformation changes of proteins and biomolecular interactions. Recently, our group discovered that the translational diffusion coefficient can be a useful and sensitive probe to detect the conformation change as well as the intermolecular interaction changes. Although many techniques, e.g., dynamic light scattering, Taylor dispersion, capillary method, NMR spectroscopy, have been developed to monitor molecular diffusion, molecular diffusion has never been considered as a time dependent property during reactions. We have been developing a method based on the pulsed-laser induced transient grating (TG) technique to detect the time-dependent diffusion. Here, we repot the time-resolved detection of protein conformation changes of a blue light sensor protein of phototropin by using the diffusion coefficient.