The aim of this study was to obtain information on the ancient material of cast bronze through an investigation of the microstructure and impurity characteristics of the casting. Three Koryo bronze coins were analyzed using an optical microscope, scanning electron microscope, and electron dispersive X-ray analyses were used to determine the composition of the specimens. The three coins had 4 phases: αphase, δphase, Pb, and impurities (Cu2S). Cu2S was found to exist near Pb or in δphase. Cu2S is the inter mediate product of copper ore refining. Therefore, the copper ore was not completely refined. To find out the characteristic of Cu2S, we melt 1)Koryo bronze coin and 2)Cu2S and Pb powder at 1273 K. The reaction between Cu2S and Pb at 1273 K yielded fine Cu and black gas, which was identified to be PbS and is presented below: Cu2S+Pb → PbS↑+2Cu.