Many studies of folding-unfolding transition of protein have been carried out for the clarification of the fundamental biological problem of protein folding, namely, how a polypeptide chain attains its native conformation. Although one of the approaches to solve protein-folding mechanisms is evidently to follow denaturation processes of protein, there is little direct evidence showing intramolecular structural changes under thermal perturbation. By using a wide-angle neutron-scattering spectrometer, we have monitored the thermal denaturation process of hen egg-white lysozyme at different pH under the unaggregative condition. The present results show that the temperature elevation gradually induces an intramolecular structural fluctuation prior to a main transition of the tertiary structure without accompanying significant heat absorption, and the multiplicity in the thermal transition observed suggests the presence of hierarchic stability depending on the structural hierarchy which would characterize an energetic landscape of folding-unfolding transition of protein.