An approach for the motion planning of an underwater manipulator is described in this paper. This approach is composed of the path planning of the tip of the manipulator and the posture planning of the manipulator. In order to execute each planning, two kinds of Genetic Algorithm is used in this approach. One is used to generate the suitable path. The other is used to generate candidates for secure manipulator’s postures. And one secure manipulator’s posture is decided from among generated candidates, by an evaluation considering the drag force.