Grain or legume having an increased content of a functional component (.gamma.-aminobutyric acid) is obtained by humidifying grain (such as rice, wheat or corn) or legume (such as soybean and adzuki bean) by forcing the grain into an air having a temperature of 50.degree. C. or more and a relative humidity of 90% or more such that a moisture content thereof rises within the range of 16.0% to 18.5%, and thereafter, drying the grain or legume.