This paper deals with an anti-sway control for a ship-mounted container crane which is disturbed by the wave-induced motions of the ship. We derive a simple dynamics of the ship-mounted container crane with an active anti-sway control system and transform it into a dynamic function for a horizontal variable on the absolute coordinate. Then we propose an control method to reduce pendulation of the spreader and compare its performance with well-known feedback linearization control in computer simulation.