PURPOSE: To improve accuracy of swing excavation width, by a method wherein the length of a swing ship hull is found through detection of a ship hull swing angle, a rudder depth, and a stopper position, and further, a swing center distance is found to display the result.

CONSTITUTION: A dredger body is provided with a ship hull swing angle detector 1, a rudder depth detector 2 and a spad position detector 3, a detecting signal from the detector 2 is inputted to an arithmetic unit 7 through converters 4, 5 and 6 to compute a length L of a swing ship hull. From the length L of the ship hull, and a ship hull swing angle θ, a swing center distance W between a longitudinal center line A passing a swing center line 10 and a cutter 12 is computed, and the computed result is displayed on a display device 8. This, even if a rudder depth or the swing central point position is varied, can find a swing distance between the longitudinal center line passing the swing central point and the cutter.