PURPOSE: To extend an auxiliary sheave without raising or lowering a boom, and heighten an operational efficiency and safety in a narrow place by supporting an auxiliary sheave structural body outside of the tip boom so that the auxiliary sheave structrual body can slide, and arranging a stopper device to prevent the slip-out of the auxiliary sheave structural body.

CONSTITUTION: An auxiliary sheave device 1 connected directly to a tip boom with a pin 12 is connected to a cylindrical or pillar shape auxiliary sheave structural body 2 on the outer side surface of the tip boom 3, and the auxiliary sheave device 1 is installed on the tip boom 3 through the auxiliary sheave structural body 2, and is made slidable in the boom shaft extending direction. A support device 4 is constituted outside of a tip boom 2 so as to be capable of supporting the auxiliary sheave structural body in both the vertical and lateral directions, and there is provided a stopper device 17 to prevent the auxiliary sheave structural body 2 from slipping out of the tip boom 3 when it slides in the boom shaft extending direction.