The demand of platinum group metals (PGM) such as platinum, rhodium and palladium is recently increasing for a catalyst for decomposition of toxic gases in automobiles. The worldwide scarcity and high price of PGM accelerate the PGM recycling. The liquid copper has been used as a collector metal in one of the major methods for the recovery of PGMs from scraps of the automobile catalyst. With the aim of the minimization of the loss of PGM into slag phase, the distribution ratios of platinum, rhodium and palladium between the molten copper and the Al₂O₃-CaO-SiO₂-Cu₂O slag under MgO saturation with Q=0.36 and 0.52 (Q=(mass%CaO+mass%MgO)/(mass%CaO+mass%MgO+mass%SiO₂)) were investigated at 1723 K in the range of the oxygen partial pressure 10⁻⁸ to 10⁻³ kPa. The copper solubility in the slag increases with increasing the oxygen potential, and liquid Cu₂O separated from the Al₂O₃-CaO-SiO₂-Cu₂O slag with the oxygen partial pressure of 10⁻³ kPa. When the distribution ratio of PGMs between the slag and liquid copper was defined as L_PGM^s/Cu=(mass%PGM in slag)/[mass%PGM in Cu], L_PGM^s/Cu for platinum, rhodium and palladium were almost constant with increasing the oxygen potential up to 10⁻⁶-10⁻⁵ kPa and increased in the range of higher oxygen partial pressure. Based on these distribution ratios, activity coefficients of Pt and Pd oxides in the slag were thermodynamically calculated. The activity coefficients increase with increasing the oxygen potential.