PURPOSE: To prevent the breakage of a natural cooling device by a method wherein an on/off valve is opened to effect double drying when the on/off valve of a natural cooling and grain collecting trough is closed naturally while the circulation of grains is switched into the circulation through only a hot air drying device when faulty circulation of grains in the natural cooling device is detected.

CONSTITUTION: Grains in the grain natural cooling and reserving chamber 1 of a natural cooling device 4 are dropped into the grain drying chamber 5 of a hot air drying device 7 through the on/off valve 3 of a natural cooling and grain collecting trough 2, then, flow down subsequently into a grain drying chamber 6 and are recirculated to said natural cooling and reserving chamber 1 to effect double drying in such a manner. In this case, an abnormality is detected when the valve 3 is closed naturally and the on/off valve 3 is controlled automatically so as to be opened whereby the drying becomes double drying. When the circulation of the grains through the natural cooling device is stopped during double drying, the faulty circulation is detected, the double drying is stopped and switched to single drying, in which the grains are circulated through only the hot air drying device 7, whereby the grain drying of single drying, in which the grains drop from the grain drying chamber 5 into the drying chamber 6, is continued.