The kinetics of conformational change in the N-terminal region of photoactive yellow protein (PYP) was studied by the time-resolved diffusion measurement. The transient grating signal that represented the protein diffusion of the ground state and pB state depended on the observation time range. An analysis of the signal based on the time-dependent diffusion coefficient clearly showed that protein diffusion changed with a time constant of 170 micros, corresponding to the pR(2) --> pB' transition. Since a previous diffusion study of N-terminal truncated PYPs had revealed that the change in the diffusion coefficient reflected the unfolding of the alpha-helices in the N-terminal region of PYP, the results indicate that this unfolding took place at the same rate as the pR(2) --> pB' transition. This demonstrates that the response of the conformational change in the N-terminal region was quite fast, probably due to changes in a specific hydrogen-bonding network of this domain.