PURPOSE: To prevent an interruption of drying operation caused by lack of fuel from being generated and further prevent either a deterioration in quality of crops or cracking at a barrel of drying machine by a method wherein when a calculated drying residual time and a drying capable time are compared to each other and the drying residual time exceeds the drying capable time, the present flow rate of fuel is reduced to a predetermined value.

CONSTITUTION: During a drying operation, a combustion control is carried out under a predetermined timing. At first, an output of a liquid level meter 29 is read and it is discriminated whether or not a residual amount of fuel in a fuel tank 29 is a predetermined value in reference to the read output value. In the case that the residual amount is the predetermined value, a residual time H1 until completion of the drying operation is calculated. Then, when a burner 10 is ignited under the present flow rate of fuel to continue the drying operation, a time H2 in which the drying can be carried out is calculated. Then, when the residual drying time H1 exceeds a drying capable time H2, an alarm of adding fuel is carried out by either a lamp or a buzzer and the like and at the same time the present flow rate of fuel is decreased to the predetermined value. Then, the burner 10 is controlled in such a way that the value may become the changed flow rate of fuel.