When a ship is required to keep the specified position with high accuracy under influence of external disturbances, dynamic positioning system (DPS) is often used in order to keep its position. It will be able to construct a control system which has no interaction among the controlled variables by using the inverse linear quadratic (ILQ) optimal servo theory. It will be also able to express the optimal feedback gain in terms of the system matrices and the design parameters. In ILQ optimal servo theory, time constant T_i as the design parameters have much influence on the performance of the control system, and the suitable values of T_i should be selected as small as possible corresponding to external disturbances. However, it is troublesome to select suitable T_i because we have to carry out numerical simulations repeatedly. In this paper, we applied the ILQ optimal servo theory to design the control system of DPS for a selfpropulsive barge. About time constant T_i , we designed the system with T_i defined automatically in response to the dimensions of controlled variables. Applying this method, it is possible to adjust the suitable values of T_i automatically. From the numerical simulations, it is found that design process taking the system with T_i defined automatically would be more useful and easier than taking usual ILQ optimal servo theory.