Dynamic birefringence and viscoelasticity measurements of bottlebrush-like polystyrene polymacromonomers having various branch (side chain) lengths were conducted over a wide frequency region covering from the glass-to-rubber transition zone to the flow zone in order to discuss effects of side chain length on dynamical stiffness of main polymer chain. Strain-induced birefringence at high frequencies was negative, similarly to linear polystyrenes, irrespective of side chain length, while birefringence in the flow zone strongly depended on side chain length. By using the modified stress-optical rule (MSOR), the complex modulus was decomposed into two components, main chain and branch components. With increasing the side chain length, the dynamical stiffness of main chain (number of repeating units per viscoelastic segment) first increased and then leveled off, suggesting that repulsive interaction between branches would be screened out over a certain branch length.