PURPOSE: To correctly detect the oscillation angle in the turning direction of a hoisting-up rope, as for a turning type crane.

CONSTITUTION: Both the edge parts of a main sheave pin 13 which supports a main sheave 6 in turnable manner is engaged in tight fitting form with a main sheave pin hole formed at the top edge part of a boom, and the pin is installed at the top edge part of the boom in both side supported state, and each load in the direction perpendicular to the sheave pin axis center which acts on the supporting points on both the sides is detected by each load sensor 15a, 15b, 16a, 16b. Further, the turning-up angle of the boom 4 is detected by a boom angle sensor 18, and the oscillation angle in the turning direction of a hoisting-up rope for the vertical line is calculated from the above-described detection value by a calculator.