A sperm diluent of the present invention contains a chelating agent such as EDTA and/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 steroid hormone and/or, a cytokine, which suppresses migration of leukocytes. By diluting frozen sperm with this sperm diluent followed by performing artificial insemination, death of the sperm before fertilization and phagocytosis of the sperm and embryos by leukocytes in the uterus can be suppressed, allowing enhancement of the conception rate and the implantation rate.