A system for mixing different kinds of grains comprises a plurality of tanks respectively containing different kinds of grains therein. Each of the tanks has a discharge port. A volumetric weight measuring device measures volumetric weights of the respective grains delivered to the tanks to input the volumetric weight signals to a computer. The computer supplies control signals based on the volumetric weight signals to drive devices for valve devices each of which controls a flow rate of the associated grain discharged from the discharge port of the associated tank. The drive devices are respectively operative in response to the control signals respectively supplied thereto from the computer to respectively drive the valve devices so as to allow the respective grains discharged by the valve devices to be controlled in terms of weight unit. The respective grains discharged by the valve devices are mixed at a mixing station.