PURPOSE: To enable accurate monitoring of the degree of verticality of an operation by a method wherein the position of a supporting body moving in the vertical direction in the three-dimentional space is detected, through the intermediary of two units of XY recorders, from a double-integration output signal of the outputs of a pair of horizontal acceleration detectors fitted to the supporting body perpendicularly to each other and from an output signal of a depth gage.

CONSTITUTION: In an underwater soft ground improving device, for instance, accelerometers 1 and 2 measuring the acceleration in the directions of axes X and Y are provided in the positions near stirring blades 3 located at the tip of a support 4 which is fixed to a bearing 6 supporting the rotary shaft 5 of the stirring blades 3. The output signals of the accelerometers 1 and 2c are sent to the X-input terminals of X-Y recorders F and G via double-integrators C and D, respectively, through the intermediary of an offset reset signal E. Meanwhile, a displacement signal in the direction of the axis Z is sent from an improvement depth gage H (a wire extension length gage, etc.) to the Y-input terminals of the X-Y recorders F and G and thus the position on of the tip of the support 4 in the three-dimensional space is detected. Accordingly, the degree of verticality of the elevation of the stirring blades 3 can be monitored and recorded accurately and simply.