The present invention addresses the problem of providing a grain thresher in which the fin opening degree of a swinging selector and the rotational speed of a blower fan are controlled to perform an efficient selection process wherein the number of times the rotational speed of the blower fan is switched is reduced to lessen the load and the control specifics are also simplified. The present invention includes a detection sensor 66 for detecting a physical quantity and a control unit 50. The control unit 50 increases the rotational speed of the blower fan 23 when the physical quantity detected by the detection sensor 66 exceeds a fan speed increase value Q2 which is a predetermined threshold and the control unit 50 increases the fin opening degree and sets the fan speed increase value Q2 to be greater than a fin adjustment value Q1 which is a predetermined threshold when the detected physical quantity exceeds the fin adjustment value Q1.
