PURPOSE: To obtain the title device rapidly responding change of culm length, properly controlling threshing depth even in false detection with floating straw, etc., by equipping a grain culm transporting path to a thresher with a threshing depth adjusting means, a threshing depth detecting means, an ear tip sensor and a culm length sensor.

CONSTITUTION: A grain culm transporting path to a thresher is equipped with threshing depth adjusting means 11-14 regulating threshing depth of a thresher, an ear tip sensor S1 of discovering ear tip position of transported grain culms at a lower side than the threshing depth adjusting means 11-14 in the grain culm transporting direction, a control means H of working the threshing depth adjusting means 11-14 based on detected information of the ear tip sensor S1, a threshing depth detecting means PM of discovering a threshing depth adjusting state of the threshing depth adjusting means 11-14, a culm length sensor S2 of discovering culm length of transported grain culm at an upper side than threshing depth adjusting positions in the grain culm transportation direction, the control means H restricts the adjusting range based on information of an ear tip sensor S1 of the threshing depth adjusting means 11-14 to a partial range of the whole adjusting range and the adjusting range is set in a shallow threshing side with increase in culm length.