PURPOSE: To make it possible for the grounding float to drive on the mud under control with the number of needed grounding floats reduced, by allowing a grounding float to play a role of a sensor float.

CONSTITUTION: The second connecting mechanism 27b is constituted from interlocking lever 26, the other end of which connected to the vibration parts 25a of the control valve for the rolling cylinder. Thus, the rolling cylinder is automatically operated so that the pitching cylinder 12 is automatically operated in such a state where the front end height of grounding float comes in a prescribed range and the rolling cylinder is automatically operated so that the grounding float 6 becomes parallel or almost parallel to the rear-front line of the machine body. The pitching cylinder 12 constitutes the connection mechanism 27 along with the rolling cylinder and the grounding float 6.