The present invention provides a method of growing spermatogonial stem cells of mammals and the like in vitro, which is characterized in that glial cell-derived neurotrophic factor (GDNF) or an equivalent thereto, and leukemia inhibitory factor (LIF) are contained in a medium (culture broth) for culturing spermatogonial stem cells. According to the method of the present invention, spermatogonial stem cells can be grown in vitro to the extent that enables use thereof for developmental engineering.