A cast multilayer film of poly (ethylene imine) (PEI) complex involving 4-dodecyloxycinnamic acid (C12OCin) was found to show layer shrinkage along the c-axis on the glass plate under UV illumination at lambda > 300 nm. The bilayer units in the stacked lamella film successively shifted from 4.7 to 4.2 nm in length, induced by the E-to-Z photoisomerization, indicating continuous changes in the layer distances. This shrinkage behavior shows a contrast to that observed in the case of an ionpair film of dioctadecyldimethylammonium cinnamate (DDAC) which exhibits no intermediate layer distance during a similar photoinduced shrinkage of the film.