PURPOSE: To automatically set a leader at a desired angle by controlling the leader angle support device based on signals from the operation section and the leader tilting angle sensor.

CONSTITUTION: Each operation section and each sensor are actuated to seek the values of x1 and x2 when they are switched on. All the relays R419, R621, R620, R722 are excited at an output signal from the comparator 18, causing each contact point to be connected. In the meantime, all the contact points of relays R823, R1126 are connected at an output signal from the comparator 19 and the direction control valve 80 is actuated to send oil discharged from pump 81 to an oil compression chamber on the contracting action side of hydraulic cylinder 5 so that the leader angle support device A is contracted. At the same time, the direction control valve 80' is actuated to contract the leader angle support device D and the point P comes right on a locus f or f'. Then the leader is positioned at a cross point of lici f and f' which represents a position of relationship x2=X=x1, i.e. a set tilting angle position. At this position, the direction control valve 80' is restored to a neutral position by a signal from the comparator 27 and the leader angle support device A' stops contracting motion.