Simultaneous measurements of stress relaxation and differential dynamic modulus were made at 20 degrees C for polyisobutylene in large shearing deformations on which intermittent small oscillations were superposed perpendicularly. Static shear strain varied from 0.1 to 1.0 while the frequency of the superposed oscillations was 0.1 Hz, which falls in the middle of the plateau zone of the polymer. The strain dependence of the differential dynamic modulus was found to coincide with the damping function in relaxation modulus which is well explained by the Doi-Edwards theory. It was concluded that non-linear behaviour in relaxation modulus results from a change in entanglement structure.