PURPOSE: To automatically control the opening of a dust-sending valve by placing an opening-controlling means to correct the opening of a dust-sending valve of a threshing chamber according to the attaching state of a reaping part of a reap-treatment part.

CONSTITUTION: An attaching state detection means detects whether the reap- treatment part 4 is provided with a reaping part 4A for single-stage reaping or with a reaping part for upper and lower two-stage reaping. When the two- stage reaping part is judged to be present, the target opening of a dust-sending valve 19 of a threshing chamber A is corrected toward the closed side compared with the case of the single-stage reaping part.