PURPOSE: To decrease aberration by disposing plural optical deflecting means in such a way that plural deflected light pass substantially through one point.

CONSTITUTION: An IDT (comb-shaped electrode) 21 has a min. central frequency and is so inclined to conducted incident light 20 that an SAW surface acoustic wave 25 satisfies Bragg conditions. The SAW 25 intersects with the light 20 at the point C and diffracted light 29 is emitted but the diffraction angle is small. The IDT 21 is so disposed that the central light of the light 29 passes through the central point B of a lens 33. Like the IDT 21, IDTs 22W24 have successively increasing central frequencies at which SAWs 26W28 are inclined with respect to the light 20 so as to satisfy the Bragg condition and intersect with the light 20 at D, F and F thus generating diffracted light 30W32. The central rays of the diffracted light 29W32 are all passed substantially through the central point B of the lens 33 in the above-mentioned way, by which the diffracted light after passage through the lens 33 is kept practically free from the influence of the error in the manufacture of the lens over the entire operating frequency band of the IDTs.