A sperm diluent of the present invention contains a chelating agent such asEDTAand/or, EGTA, which forms a complex with a calcium ion, in a base diluent.Further,the sperm diluent contains an immunosuppressive factor such as a steroidhormone and/or,a cytokine, which suppresses migration of leukocytes. By diluting frozen spermwiththis sperm diluent followed by performing artificial insemination, death ofthe spermbefore fertilization and phagocytosis of the sperm and embryos by leukocytesin theuterus can be suppressed, allowing enhancement of the conception rate and theimplantation rate.