Hydrodynamic forces including a wave drift damping for a slow drift motion of a four-column platform are investigated, both experimentally and theoretically. It is well known that the 2nd order hydrodynamic forces acting on the marine structures composed of multiple cylinders in both current and waves are significantly influenced by the interaction between cylinders, and extremely vary with the wave number ka. We measured the 2nd order wave forces and moment acting on a four-column platform model, in both slow current and regular wave trains of various incident angles. The experimental results were compared with the results of semi-analytical solutions based on the potential flow theory. In this solution, the interactions between cylinders are represented by additional waves emitting form adjacent cylinders towards the cylinders under consideration, and the drift forces as well as the yaw moment are evaluated by the far field method. It was confirmed that the yaw moment is significantly induced by oblique waves and current, and that those phenomena are well explained by the theory.