This invention is characterized by a brain function improving preparation containing theanine not less than 50 .mu.M. It was confirmed that theanine could increase intracellular Ca.sup.2+ concentration by acting on NMDA receptors even at a low dose of about 50 .mu.M. Thus, the brain function improving preparation of this invention was shown to be capable of causing plastic change in neurons or neural circuit. Since theanine has been approved as a food additive and being consumed daily, its safety has no problem. Moreover, theanine passes the blood-brain barrier relatively easily and, thus, it is effective as an oral preparation.