Dynamic viscoelasticity and birefringence of two poly-(ionic liquid)s, PC4VITfO and PC4VITFSI, are investigated to clarify the molecular origin of viscoelastic response of PILs. According to a previous study, PC4VITFSI having larger counterions shows a broader viscoelastic relaxation spectra around the glass-to-rubber transition zone than PC4VITfO. The rheo-optical data were analyzed with the modified stress-optical rule: The complex modulus for PC4VITfO was separated into two components, the rubbery and the glassy component, similarly to the ordinary amorphous polymers while for PC4VITFSI system an additional component was necessary in addition to the two ordinary components for a reasonable separation. From the frequency dependence, the additional component was attributed to the sub-Rouse mode of chain which is enhanced by the counterions decreasing the interchain interactions.