The present invention enables efficient suspension culture of stem cells in a serum-free medium by comprising a step for quickly forming a homogenous aggregate of stem cells, and provides a method of selectively inducing the differentiation of nerves from a stem cell, a method of forming a cerebral cortical nerve network in vitro, and a method of producing a steric structure of a brain tissue in vitro, as well as a method of producing hypothalamic neuron progenitor cells, comprising culturing pluripotent stem cells as a suspended aggregate in a serum-free medium that substantially does not contain a Nodal signal promoter, a Wnt signal promoter, an FGF signal promoter, a BMP signal promoter, retinoic acid and an insulin, and isolating hypothalamic neuron progenitor cells from the culture.