The paper is concerned with a dynamic positioning control of a moored floating platform model. Moorings are used to maintain the position of the platform against quasi-constant external forces induced by current and wind. The control purpose is to suppress the slow drift oscillation due to drifting force in irregular waves using thrusters so that they do not respond to linear exciting force in wave frequency range. Control of motions in wave frequency region has to be ignored because the amplitude of those motions is smaller than those of the slow drift oscillation and enormous thruster power is needed to control them. The problem is formulated in the framework of H_∞ control, and is solved using LMI (Linear Matrix Inequalities) Control Toolbox on Matlab ; some successful results are shown.