In direct hoisting work in suspension bridge construction, since a cable crane directly hoists a bridge beam block put on a barge on the sea, precise fixed position holding function is required for a barge. The control system was then designed on the basis of an ILQ control theory, and the effect of change in time constant as design parameter on fixed position holding performance was studied. In addition, the critical disturbance for fixed position holding control was studied through numerical simulation under various disturbance conditions using the above designed control system. As a result, in the design of control systems on the basis of an ILQ control theory, the precise control system could be designed by diminishing, in particular, the time constant for state variable around a z axis among time constants according to the extent of disturbance. The control performance was largely affected by response delay period from sensing of disturbance to beginning of control. 3 refs., 7 figs., 2 tabs.