The Takada Granodiorite, in the central part of the San’in batholith in southwest Japan, is thought to have undergone prolonged magmatic activity based on zircon U–Pb dating. The pluton consists mainly of hornblende–biotite granodiorite that shows textural evidence for mingling with mafic magma (Ogi Quartz diorite). Oscillatory zoning in minerals (e.g., anorthite content in plagioclase, Zr content in titanite, and Ba content in alkali feldspar) and remelting textures are common in the Takada Granodiorite. Hornblende and titanite geothermobarometers indicate emplacement pressures of 180–200 MPa with temperature fluctuations near the solidus. Moreover, whole-rock Rb–Sr isotopic compositions did not form a well-defined isochron. These observations suggest multiple injections of granitic magma with isotopically heterogeneous source material in addition to mafic magma, which account for the prolonged magmatic activity that produced the Takada Granodiorite.