The conformational relaxation of single polymer chains in uniaxially elongated poly(methyl methacrylate) (PMMA) films were examined by direct observation using scanning near-field optical microscopy (SNOM) and excitation polarization modulation microscopy (EPMM). The relaxation behavior of the individual PMMA chains was investigated in the binary blend systems consisting of high and low molecular weight components. The whole chain dimension and the segmental orientation were evaluated for high molecular weight long chains from SNOM and EPMM images, respectively. At the early stage of the stress relaxation process, not only the segmental orientation but also the whole chain dimension of the long chain decreased faster in matrices with short chains. This indicates that the relaxation at the whole chain scale is accelerated by disentanglement, which is caused by the motion of the short surrounding chains. The fast relaxation in the whole chain dimension cannot be explained by the constraint release picture in the theoretical models, which assumes that the disentanglement only activates the local motion of the long chain.