Thin films of amorphous silicon-tin alloy (Si1-xSn2) have been prepared by sputtering silicon and tin targets with argon in radio-frequency magnetron sputtering equipment. X-ray diffraction reveals that the film of x < 0.28 is amorphous but that crystalline beta-Sn is formed in the amorphous film for x greater than or equal to 0.28. The optical energy gap is expressed by E-g = 1.4 - 3.1x [eV] for x < 0.28. The equation also correlates the pseudo-optical energy gap for the SiSn-beta-Sn composite film. Annealing the film for x < 0.28 forms crystalline beta-Sn and shows a small value of the pseudo-optical energy gap.