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This paper presents a basic research on the problem of how to install a flexible structure whose elastic response must be taken into account during the installation in the deep-sea, using an active control technology. This technology will be needed because the trend of the target water depth of the underwater development is becoming increasingly deep so that the facilities and structural components must be installed by remote operation. In this paper, a model of a flexible structure is proposed first, then the equations of motion and the control systems are formulated, and after the simulation, the whole system is realized and checked with the actual structure model. The equations of motion of the structure becomes a coupled form of rigid body motion and elastic response, and it is simplified further assuming that the roll and pitch motion or velocities are small. It is impossible to control infinite sets of modes of the structure, so that the control modes are extracted by using the modal filter and formulated by IMSC(Independent Modal Space Control). DVFB(Direct Velocity Feedback Control) is used to prevent the uncontrolled modes from excitation due to the spillover. This technique is called LAC/HAC approach, which makes the system robust against the nonlineality and uncertainties in the dynamics of the structure. The experimental model is a frame structure which is consisted of 25 rigid body cylinders connected by 80 flexible beams. It is also almost neutrally buoyant. 3 thrusters, 7 variable buoyancy type actuators and 11 ultrasound acoustic displacement sensors are attached to the model. The model is propelled along the trajectory by the thrusters with its elastic response, roll and pitch motion controlled by variable buoyancy actuators. During this operation, elastic response of the model is successfully controlled and transient response of the model is well damped.