An objection of the present invention is to provide an agent for preventing or improving decline in brain function such as decreased perception ability, decreased memory learning ability, decreased thinking ability, decreased concentration, decreased attention, decreased judgment ability, depression, and decreased exercise performance caused thereby. According to the present invention, an agent for protecting brain neuronal cells, comprising citrulline or a salt thereof and glutathione or a salt thereof as active ingredients as well as an agent for preventing or improving decline in brain function, comprising citrulline or a salt thereof and glutathione or a salt thereof as active ingredients is provided.
