PURPOSE:To improve detection precision by projecting part of a supply roll into a grain tank wherein grains are stored, providing a grain supply guide which appears and disappears freely above the supply roll, and also providing a water detecting device outside the grain tank. CONSTITUTION:Part of the supply roll 1 which has a spiral on its outer peripheral surface is projected into the grain tank 2 where grains are stored, the grain supply guide 3 which appears and disappears freely is provided over the supply roll 1, and the water detecting device 4 which receives grains fed by the supply roll 1 and detects their grain is provided outside the grain tank 2. In this case, the amount of grains which are supplied is varied by adjusting the insertion position of the grain supply guide, so the number f supplied grains is stabilized according to the kind, state, etc., of the grains and the detection precision is improved.
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