PURPOSE: To prevent danger in turning on very wet ground, by providing a detector which is actuated by an operating lever for the turning on the very wet ground such as a paddy field and by relating the detector to the speed reduction control of a position controller to reduce the speed in the turning and restore the speed to the original level after the turning.

CONSTITUTION: When a limit switch 51 is turned on by putting a left steering lever 36 in a transverse groove 35b for left turning on very wet ground, a relay R3 is operated so that contacts R3a, R3a' are turned on. Consequently, a motor 56 is reversed in the direction of deceleration and a stepless transmission is put in a speed reducing position to control a combine to a low speed. When the lever 36 is put out of the transverse groove 35b at the end of the turning on the very wet ground, the switch 51 is turned off and the relay R3 is deenergized so that the contacts R3a, R3a' are turned off and contacts R3b are turned on. Since a control plate is in a low speed position, a speed increaing control state is provided. A position controller 54 is put in a higher speed position corresponding to the prescribed high speed position of a speed change lever 53, to restore the combine to the original speed.