Those provided here are an Activin A-containing solution for making inhibitory neuron progenitors proliferate, a method for culturing the inhibitory neuron progenitors using this solution, a method for separating the inhibitory neuron progenitors and the inhibitory neurons using an Activin A receptor Acvr1 expression as an index, a method for transplanting thus separated cells to the brain surface, and a method for screening a factor for making the inhibitory neuron progenitors proliferate.