PURPOSE: To eliminate the influence of boom motion by transmitting the rocking of a wind-up rope to a sensor atop a boom through a detecting element, and detecting right-left and front-rear angles of the rope to the boom and the hoisting angle of the boom and calculating the deflection angle of the rope to the perpendicular direction.

CONSTITUTION: The detecting element 11 is provided between the sensor body 10, fixed atop of the boom 1 so that the boom 1 is horizontal at a specific reference hoisting angle, and the wind-up rope 2. Further, the angle of rotation of the detecting element 11 which rotates according to the rocking of the rope 2 is detected separately as the right-left angle and front-back angles of the boom. Then, a boom hoisting angle detector detects the hoisting angle of the boom to calculate the difference from the reference value, thereby calculating the deflection angle of the rope 2 to the perpendicular direction from the difference in the hoisting angle and the detected value of the main body 10.