PURPOSE: To improve work efficiency by a method wherein when the opposite ends of a supporting pin inserted into the lower end of a leader are supported by a bracket on the side of a work machine, an open groove opened in a pin diameter direction is provided in a fitting part of at least one of the leader and bracket.

CONSTITUTION: While a supporting pin 6 is mounted to one of a leader bracket provided on a work machine and a leader 2, the other one of the bracket and leader is moved along an open groove 21c to engage the pin 6 with the groove 21c or to disengage the pin from the groove, thereby engaging the leader 2 with the bracket or disengaging the leader from the bracket. Next, as required, a limiting member is mounted to the opening of the groove 21c to prevent the pin 6 from slipping from the end of the opening of the groove 21c. And when the groove 21c is provided so as to be opened at the lower end surface of the leader 2, if the leader 2 is lifted or lowered while the pin 6 is mounted to the bracket, the pin 6 is engaged with the leader 2 or disengaged therefrom through the groove 21c. As a result, the leader can be disassembled or assembled by means of one crane and work efficiency can be improved.