1. A method of increasing neuronal axon elongation, comprising: a treating step of treating neuronal growth cone with a regulatory factor to reduce macropinocytosis caused by a repulsive axon guidance molecule, the treating step inducing promotion of the axon elongation as a result of reducing the macropinocytosis, wherein the regulatory factor contains a nucleic acid that encodes for Syntaxin 1 B protein, wherein the treating step comprises an administering step of administering an effective amount of the regulatory factor to a human or non-human animal, wherein the human or non-human animal is one suffering damage to a nervous system thereof or is to undergo an operation on its nervous system thereof, and wherein the regulatory factor is administered by a method comprising direct injection into a vicinity of a nervous system to be treated.