This paper describes a control system of grain return flow by using an impact-type grain flow rate sensor in a head-feeding combine. A control system was used to adjust the clearance of the chaff sieve according to the measured grain return flow. This control system successfully regulated the grain return flow at a desired flow rate. In addition, we measured the quality of clean grain in the grain tank and the grain loss from a suction fan, which was proportional to the grain return flow, using indoor threshing tests. Ratios of damaged grain, grain with rachis-branch, unthreshed grain head, and grain loss from the suction fan were less than 1.31%, 2.77%, 1.44%, and 3.98%, respectively.