PURPOSE: To obtain an invariably accurate water value by calculating the area of a detection voltage waveform detected by a detection roll and the area of a rectangle formed of the waveform width of the detection voltage waveform as a base and a parallel line running on the peak value.

CONSTITUTION: Grains supplied from a conveyor roll 1 to the detection roll are pressed and crushed by the detection roll, and the voltage waveform (a) of crushed grains is detected. The area of the detection voltage waveform (a) is calculated and the area of the rectangle (b) formed of the waveform width of the detection voltage waveform (a) from the start to the end as the base and the parallel line running on the peak value is calculated. The ratio of the calculated area of the detection waveform (a) to the calculated area of the rectangle is calculated to decide that the water value of refined grains is detected when the calculated ratio is larger than a specific value or that the water value of green rice is detected when the ratio is less than the specific value, thereby performing green rice detection control. Thus, when the water value of green rice is detected, a cut process is carried out to obtain the invariably accurate water value.