The relaxation behavior of photo-induced anisotropy has been investigated in a side-chain liquid crystalline(LC) poly(malonic ester) (PCN). Abnormal phenomenon, increment of the induced birefringence, in decaying process was observed just after the pumping beam polarized perpendicularly to the recording beam was illuminated in the relaxation process. This is difficult to explain using the present models, the thermal cis-trans isomerization or the free volume effect for the relaxation process. We suggest a new model considering the elastic force between the side-chain and the backbone in the polymeric film. According to our model, the backbone can be reoriented together with the side-chain, and fast relaxation is associated with the elastic force.