A surface treatment composition and method for removing Si and reduced metal salt produced during etching of an aluminum die cast material ("ALDC material") without generation of nitrogen oxide (NOx) or hydrogen fluoride (HF). In surface treatment of an ALDC material containing Si, Fe, Cu, Mn, Mg, Zn and Ni, the surface treatment composition for removing Si and reduced metal salt from the surface of the ALDC material after etching comprises hydrogen peroxide 300 to 950 g/l, fluorine ion-containing inorganic salt 1 to 300 g/l and balance water. The surface treatment method for removing Si and reduced metal salt from the surface of an ALDC material after etching comprises the step of dipping the ALDC material into the above described surface treatment composition. The aforementioned surface treatment composition effectively removes the Si and reduces metal salt impurities from the surface of the ALDC material without any problems such as NOx or HF gas which is harmful to the human and waste water treatment. Further, residue oil is also removed from the ALDC material.