PURPOSE: To enforce an inexpensive construction work and to prevent a wide range lawn from being damaged by running a main body by two back and forth wheels by controlling the steering angle of a steering wheel so that the main body may not fall according to the inclined angle determined by an in clined angle calculating device.

CONSTITUTION: A main body runs by the two wheels of a drive wheel 3 and a coupled driving wheel 26. When the main body is inclined, a gyro sensor 10 detects the inclined angle and a steering wheel control circuit 19 rotates a steering wheel 26 according to the inclination and corrects the inclination of the main body. When the number of rotation of a driving motor 2 or running speed is below a prescribed value, the main body is thrown out of balance because it runs by two wheels and has the possibility of falling down in either way of left and right sides. Therefore, when the running speed becomes below the prescribed value, an auxiliary wheel 11 is controlled to be put out by an auxiliary wheel putting in and out circuit 22. As the main body runs by two wheels, it can be done by a running route, the enforcement expense of the construction work is inexpensive and a wide range lawn can be prevented from being damaged.