Characteristics of electron injection from the electrode in the organic devices is generally discussed based on the organic/electrode interface without any metal penetration. In this study, the cross-sectional structure of the interface of tris(8-hydroxyquinolinato)aluminum (AlQ), which is a typical organic semiconductor, and facing-target sputtering fabricated Mg:Ag alloy electrode is investigated. The penetration of Mg to a depth of several nanometers in the AlQ underlayer is observed. This indicates that the charge injection in organic devices with sputtered electrode should be discussed based on the organic/electrode interface with metal penetration.