Abstract: An automatic control device for a boundary plate of a grain separator for separating a grain mixture into different grains. The grain separator has a separator plate provided with a roughened surface and mounted at inclinations in both of longitudinal and lateral directions with respect to the horizontal plane. A lateral movement is imparted to the different grains on the separator plate by a lateral oscillation given to the separator plate or by a lateral jet of air jetted from a numerous small aperture formed in the separator plate, so that the flow fluxes of different grains are deflected away from each other due to the difference in the physical properties such as specific weight and/or friction coefficient, so that the mixture of different grains are separated.