PROBLEM TO BE SOLVED: To provide effective means for treating and improving male infertility as a novel application of a somatic cell, and also provide means for improving a success rate and efficiency of artificial insemination in the field of livestock farming and breeding.SOLUTION: A cell selected from the group consisting of adipose tissue-derived stem cells, bone marrow-derived stem cells, dental pulp-derived stem cells and fibroblasts and sperms are co-cultured to activate the sperms. The activated sperms are used for treating male infertility, artificial insemination, in-vitro fertilization or the like.