PURPOSE: To make a structure dense to a degree equal to that of a forged product and also to improve strength by applying hydrostatic plastic working to cast aluminum foil in a high-pressure vessel by means of a fluid such as water.

CONSTITUTION: A foil prepared by casting molten Al is previously subjected to shot peening treatment, by which small dents 11 and inlets 13', 13 communicating with blowholes 12 in the foil surface are filled up. Subsequently, the Al foil 1 is placed in a high-pressure vessel 3 and then a fluid 2 for pressing, such as water, is also poured into the vessel 3. Fluid such as water in a tank 10 is pressed by means of a high-pressure pump 8 via a pipe 7 and a nipple 6 into the high-pressure vessel 3, and a static pressure of at least 1,500W2,500kg/cm2 is uniformly applied to the whole surface of the Al foil 1, by which a structure having a denseness equal to or higher than that of a forged product is formed and, as a result, the strength of the Al foil 1 can be remarkably improved.