The acoustoelasticity with ultrasonic transverse waves was applied to analyze an elastic-plastic state of a shrink-fit specimen of SS41 steel. The specimen was made by inserting a cooled circular disk into a circular hole in a heated square plate. Before and after shrinkage fitting, two transverse velocities and polarization directions were measured by the sing-around method and spectrum method. The Stress distributions were obtained using these data applied to the acoustoelastic law, and it was confirmed that the elastic-plastic boundary could be determined from the distribution profile of the difference between the two transverse velocities, and also that, on diagonal lines of the specimen, the principal stress difference could be determined nondestructively. Comparison of the ultrasonic results with strain gauge results showed that velocity change due to plastic deformation was significant in the plastic region of the outer plate. Although confined to the velocity difference, such changes were experimentally verified.