PURPOSE: To permit manufacture of an element which consumes less electric power at a lower cost by forming a thin high-polymer piezo-electric film by coating on a substrate.

CONSTITUTION: The thin high-polymer piezo-electric film is formed by coating on the substrate. P(VDF-TrFE) is more particularly preferable as the material to form the thin high-polymer piezo-electric film. The thin piezo-electric film is obtainable with the heat treatment alone without poling if the thin film of P(VDF-TrFE) is formed by coating on the satisfactorily smooth substrate 9. More specifically, taking-out of diffracted light at a low scattering loss without disturbing the wave front of guided light is permitted if the acousto-optical light deflecting element is manufactured by using the thin high-polymer piezo-electric film 11 formed by forming the thin film of such high-polymer material on the substrate 9 having a satisfactorily smooth surface by coating, then subjecting the film to a piezo-electric treatment.