To prevent beforehand a contracting rope from being extended more than an allowable level thereof and broken, which is caused by excessive tension acting on the contracting rope, and to prevent beforehand a contracting sheave from being damaged.

In a boom extension/contraction mechanism, damper means 12 that extends by receiving tension acting on the contracting rope 7 which is higher than a set value to suppress the tension acting on the contracting rope 7 within an allowable range, is provided between a mounting position of the contracting rope 7 and a proximal end part of the contracting rope 7 in a tip of a boom 2 on a proximal part side. Lock pin plunging detection means for detecting plunging of a lock pin 35 into a lock pin hole 36 is provided in the boom extension/contraction mechanism to prevent a contraction operation of the boom 3 in an intermediate position by an actuator 9 while the lock pin 35 plunges into the lock pin hole 36.