1. A method for producing isolated neural stem cells, wherein the method comprises: (A) culturing undifferentiated primate embryonic stem cells in suspension in the presence of an astrocyte conditioned medium thereby forming a stem cell sphere (SCS) comprising a neural stem cell layer, a pre-neural stem cell layer, and an embryonic stem cell layer and (B) adhering the stem cell sphere (SCS) obtained in step (A) to an adhesive culture substratum carrying a cell adhesion molecule and culturing the stem cell sphere (SCS) in the presence of basic fibroblast growth factor (bFGF) and/or epidermal growth factor (EGF), and (C) obtaining neural stem cells as they migrate from the SCS.