Mesoscopic structural changes of an ovalbumin solution by heating and adding NaCl have been investigated with a small-angle neutron scattering method. In the natural solution, a broad peak at q = 0.057 Å-1, which disappeared by adding NaCl, indicates the existence of an electrostatic long-range interaction between the ovalbumin globules. Along with the broad peak, a prominent intensity increase in a very small q region was observed on heating except for the initial and final stages, indicating the coexistence of a disordered structure of the denatured ovalbumin and a regular-interspacing structure of the natural ovalbumin globules. Though the macroscopic feature in the final stage, whether the solution forms a gel (10 wt %) or not (5 wt %), strongly depended on the concentration of the ovalbumin, the scattering profiles showed a common characteristic feature: the appearance of a new peak around q = 0.023 Å-1, which indicates the emergence of another regular structure.