A thin film of an amorphous alloy comprising (i) at least one element selected from Fe and Co, (ii) at least one element selected from Pt and Pd and (iii) at least one element selected from (a) 3d transition elements other than Fe and Co, (b) 4d transition elements other than Pd, (c) 5d transition elements other than Pt, (d) light rare earth elements, (e) Group III B elements, (f) Group IV B elements, (g) Group V B elements, and (h) Group VI B elements, and the film having an easy axis of magnetization perpendicular to the film face. The thin film has excellent magnetooptical characteristics including increased coercive force and large Kerr-rotation angle and Faraday-rotation angle, and is excellent in resistance to oxidation and in consequence the coercive force and Kerr-angle of the film do not substantially change with time. The film has a high reflectance as well.