The present invention provides a manufacturing method of leguminous plants having increased content of functional component, wherein the leguminous plants are raw materials. The manufacturing method comprises the following steps of: a humidification step of humidifying a raw material； and a dehydration step which is performed after the humidification step of drying the raw material. In the humidification step, the raw material is humidified by exposing it to highly humidified air such that the moisture content of the raw material rises within the range of 16.5% to 18.5%. In the dehydration step, the dehydration is performed by reducing the temperature of the air used for the dehydration in a multi-stage or continuous gradual way, so that the moisture content of the raw material recovers to an original state before the humidification step. An object of the present invention is to provide a method of adding water to leguminous plants to increase the amount of gama-aminobutyric acid contained in the leguminous plants without controlling the addition amount of water or stirring the legume during addition of water.낟알 (쌀, 보리, 옥수수 등) 또는 콩과 식물 (대두, 팥 등) 에 50℃ 이상이고, 또한, 상대 습도가 90％ 이상인 공기에 접촉시켜 가습하여 그들의 수분을 16.0％ ∼ 18.5％ 의 범위로 한다. 이와 같이 가습한 것 후에 그들 낟알 또는 콩과 식물을 건조시킨다. 이렇게 하여, 기능성 성분 (γ-아미노부티르산) 의 함유량을 증가시킨 곡물 또는 콩과 식물이 얻어진다.
