The present invention provides an agent for promoting neuronal differentiation of neural stem cells, which contains a p38 inhibitor. In addition, the present invention provides a method of promoting neuronal differentiation of neural stem cells, which includes cultivating neural stem cells in the presence of a p38 inhibitor. Neural stem cells are gliogenic or neurogenic. The p38 inhibitor is a nucleic acid which hybridizes to DNA or mRNA encoding p38 under physiological conditions, thereby inhibiting the transcription and/or translation thereof, an expression vector of the nucleic acid, a low molecular p38 inhibitor and the like.