For the manufacture of the CuGaS2, Cu, Ga and S were vapor-deposited in the named order. Among them, Cu and Ga were vapor-deposited by using the Evaporation method in consideration of their adhesive force to the substrate so that the composition of Cu and Ga might be 1 : 1, while the surface temperature having an effect on the quality of the thin film was changed from R.T.[℃] to 150[℃] at intervals of 50[℃]. As a result, at 300[℃]of the Annealing temperature, their chemical composition was measured in the proportion of 1 : 1 : 2. It could be known from this experimental result that it is the optimum condition to conduct Annealing on the CuGaS2 thin film under a vacuum when the CuGaS2 thin film as an optical absorption layer material for a solar cell is manufactured.