A V.gamma.9V.delta.2 T cell proliferation agent includes at least a bisphosphonate, interleukin 2, and interleukin 18. Since IL-18 has properties that improve cell viability by inhibiting cell death, IL-18 is presumably capable of acting as a cofactor for the bisphosphonate so as to significantly increase the effect of V.gamma.9V.delta.2 T cell proliferation by the bisphosphonate and the IL-2. This allows providing a V.gamma.9V.delta.2 T cell proliferation agent capable of growing V.gamma.9V.delta.2 T cells with a proliferated efficiency significantly high compared to conventional methods so that the proliferated V.gamma.9V.delta.2 T cells have a high antitumor activity and high cytokine productivity.