The complex Young’s modulus, E(*)(omega), and the complex strain-optical coefficient, O-*(omega), were measured for polyisobutylene (PIE) over a wide temperature range near and above its glass transition temperature. The master curves could be constructed well for each function with the method of reduced variables. The shift factor, alpha(T), for E(*)(omega) is the same as that for O-*(omega). The ratio of the imaginary parts of O-*(omega) and E(*)(omega), O "(omega)/E "(omega), takes an extremum, which has never been observed for other polymers. The relation between O-*(omega) and E(*)(omega) cannot be described by a modified stress-optical rule (MSOR) which has been found valid for various polymers. The basic concept of the MSOR, i.e., the chain orientation and the orientation of flat monomer units in the stretch direction, is not sufficient to describe the behavior of PIE and another origin of stress, presumably due to the fluctuation of local stress, should be included. This term does not contribute to the birefringence. The main maximum of tan 6 is ascribed to the relaxation of the chain orientation in contrast with many other polymers, such as polyisoprene and polycarbonate, for which the maximum of tan delta is ascribed to the rotational relaxation of monomer units.