To accurately measure a suspending length with a simple constitution for improving vibration control accuracy and transferring efficiency.

A suspension point A of a hoisting cable 4 and a suspension point distance H between a lifted cargo 6 and a ground contact face B are derived beforehand, and a load applied on the hoisting cable 4 is detected. Dynamic lift off of the lifted cargo 6 is detected by confirming that the detected load exceeds a set load larger than the load in empty state. Regarding a point of detecting the dynamic lift off as an original point for winding, a winding length C of the hoisting cable 4 is detected, and the suspending length L is derived by subtracting the winding length C from the suspension point distance H.