Polymers that exhibit their functions by light have been studied for a few decades because they enable device miniaturization, energy saving, and precise signal control. Polymers based on azobenzene, diarylethene, etc. are the pioneers, and many examples of light-driven motors and artificial muscles have been reported. On the other hand, cinnamic acid, which is a constituent of lignin in natural wood, also exhibit the function by ultraviolet (UV) rays, so that it has been applied to polymers. The deformation mechanism of these cinnamate-based polymers has not been clarified because the two reactions of double bond cis-trans isomerization and [2+2] cycloaddition occur almost simultaneously. Since the mechanism has not been clarified, its use as a photodeformable material has not received as much attention as the above-mentioned azobenzene and diarylethene.