PURPOSE: To enhance the weighing accuracy in quantitative discharge, by altering a minute amount discharge start wt. corresponding to the supply flow amount from a grain sorter.

CONSTITUTION: The quantity of the whole grain of rice supplied to a hopper 2 from a grain sorter 1 is set by a supply flow amount setting means 8. Next, when the supply flow amount to the hopper is low, a minute amount discharge start wt. is determined to a reduced value as compared with that when said flow amount is much on the basis of the output signal of a supply flow amount setting means 8 by a minute amount discharge tart wt. determining means 9. In the next step, the determining means 9 is connected not only to a wt. measuring device 10 equipped with a balance stand 6 but also to a shutter opening degree control means 11 and the actual wt. of the grain received in a container 5 is compared with the previously determined minute amount discharge start wt. and judged by said means 11 and, when both wts. coincided, a control signal for moving a shutter 4 from a full open state to a half open state is outputted by a shutter driving means 7. By this method, the discharge flow amount at the discharge time of a minute amount becomes constant and accurate quantitative discharge according to an initial target can be always performed regardless of the efficiency of the grain sorter.