To make an accurate gain moisture value always detectable, by calculating an adding temperature from the detected temperature values of hot air and outside air and, at the same time, the temperature of grains from the adding temperature and outside air temperature, and correcting the moisture value of the grains based on the grain temperature.

When grain drying work is started by igniting a burner 3, the moisture value of the grains to be dried is measured, but the moisture voltage value of each grain is read to a controller by turning on the output of a moisture motor 30 installed to a moisture sensor 2. In addition, the outside air temperature and hot air temperature are respectively detected by means of an outside air temperature sensor 51b and a hot air temperature sensor 51a, and the temperature of the grains is calculated from the detecting signals of the sensors 51b and 51a. Then the moisture correcting value of the grains is calculated from the grain temperature and the moisture value of each grain is corrected. When a prescribed number of grains (32 grins) is measured, the output of the moisture motor 30 is turned off and the average moisture value of the grains is calculated and displayed by means of a displaying means.