Abstract: A problem of this invention it to provide a method for differentiate a primordial germ cell into a functional GV stage oocyte by in vitro culture. This invention relates to a method for differentiating a primordial germ cell into a functional GV stage oocyte by in vitro culture, comprising: (a) a step of producing a secondary follicle by culturing the primordial germ cell and supporting cells adjacent to the primordial germ cells under conditions that eliminate the effects of estrogen or a factor having a similar function to estrogen; (b) a step of partially dissociating cells between a granulosa cell layer and a thecal cell layer, wherein an oocyte, the granulosa cell layer, and the thecal cell layer constitute the produced secondary follicle; and (c) a step of differentiating the oocyte into a functional GV stage oocyte by culturing the oocyte, the granulosa cell layer, and the thecal cell layer that constitute the secondary follicle in a medium containing a high-molecular-weight compound.