Photoinduced birefringence in a poly(malonic ester) containing two symmetrical p-cyanoazobenzene groups has been investigated using two pumping beams with different wavelengths of 365 and 488 nm,. which are assignable to the absorption wavelengths of trans and cis conformers of azobenzene, respectively. We observed unexpected photoinduced birefringence even by the pumping beam of 488 nm as large as that by 365 run. To describe such specific phenomenon, we measured the magnitude and direction of the induced birefringence. by means of a new experimental setup, adding lambda /4 waveplate between the polymer film and the analyzer of conventional a pair of crossed polarizers setup. The measurement of the photoinduced phase retardation by simultaneous illumination with two perpendicularly polarized beams of 365 and 488 nm supports the fact that there is another process for the photoinduced birefringence besides a typical trans-cis photoisomerization of azobenzene.