PURPOSE: To achieve the rationalization of a grade and the enhancement of accuracy by reflecting the size distribution state of uniform screened grains at the time of the graduation of grains.

CONSTITUTION: A sample is subjected to dehulling treatment by hulling rolls 3, 3 and sorted into uniform grains and refuse grains by a main sorting cylinder 6. The screened grains and refuse grains are weighed by a weighing device 7 and the screened grains are supplied to an optical sorter to exclude foreign matter and the screened grains are sorted into N grades corresponding to large, medium and small particle diameters by a screened grains sorter 11. A weighing device 18 weighs the wts. of the screened grains sorted into N grades at every grades. A grade judging device 20 grades samples on the basis of the respective measured values of the weighing devices 7, 18.