PURPOSE: To shorten a drying time of grains, to reduce a cost, and to improve a drying speed by a method wherein drying is performed after unriped grains are removed by a grain size selecting device according to an amount of unriped grains mixed in grains intended to be dried and a drying mode when the moisture content of the grains is high.

CONSTITUTION: A grain drier 1 is formed such that a transfer bucket 12 and a ceiling plate 13 are disposed to the upper part of a machine wall 11 and a grain storing chamber 14 is formed below the ceiling plate 13. A grain drying chamber 15 is disposed below the grain storing chamber 14 and between discharge chambers 16 on both sides and a blast chamber 11. Further, a delivery valve 18 and a grain collecting bucket 19 are disposed below the grain drying chamber 14. A grain side selecting device 3 is disposed to the outside part of the machine wall 11. In this constitution, when a quantity of mixed unriped grains is high and a dehumidified air drying mode or a low temperature constant hot blast drying mode is selected, unriped grains are removed for disposal by the grain size selecting device 3 when a grain moisture content is high. Meanwhile, when mixture of unriped grains is low, the grain side selecting device 3 is brought into a stop.