PURPOSE: To economize in energy and smoothen the drying operation by providing a course of air ducts connecting the exhaust vent of a main drier to the hot air inlet of a preliminary drier, by connecting the grain outlet of the latter to the grain inlet of the former, and by controlling the air pressure in the air ducts by the use of a pressure sensor.

CONSTITUTION: When the preliminary drying has been started, an air pressure sensor 3 functions to sense the air pressure in a course of air ducts connecting a main drier 1 to a preliminary dire 2; a fall of the air pressure below the atmospheric pressure, causing a deformable plate 42 to assume the position represented in solid lines and a signal from a low pressure-detecting switch 43 to be outputted, makes an exhaust fan 23 run at an increased number of revolutions; contrarily, the exhaust fan 23 runs at a decreased number of revolutions when the deformable plate 42 has assumed the position of the dotted lines and a high pressure-detective switch 44 has outputted a signal. This control of the revolution of the exhaust fan 23 prevents dust from being raised because of a high air pressure and an exhaust-guiding duct 21 from being deformed because of a low air pressure.