PURPOSE: To enable manual operations in case of trouble with a sensor, etc., by detecting rolling-manipulated variable using a manual switch in a rolling- controlled tractor.

CONSTITUTION: A ground working machine of a tractor is rolling-controlled so that output from a sensor (C) for detecting inclination against the ground may agree with a set value of a right and left inclination-setting device (I). In case of a manual operation, a control-transfer switch (SW1) is adjusted to manual and the operation can be carried out using a manual switch (SW2). The manual operation is carried out so that a manipulated variable of the switch (SW2) may agree with output from a stroke sensor (S) of a lift rod. In case of absence of output from the sensor (S) by some failure, the lift rod can be controlled so as to be expanded or contracted corresponding to the manipulated variable of the switch (SW2).