The objective of present paper is to clarify the charactaristics of separated flow around a submerged horizontal cylinder with rectangular cross section in regular waves and steady currents. The hydrodynamic forces on the submerged cylinder were calculated by the discrete vortex method. The numerical results were compared with those obtained by other numerical method so that the accuracy of the present method was confirmed. The experiments in currents and in waves-currents were carried out and the effects of free surface and of wave-current interaction on the hydrodynamic forces and the strauhal number were discussed. Moreover, the numerical results obtained by the present method were compared with the experimental results so that its applicability was clarified. The results of the visualization experiments were compared with the flow pattern computed by the present method and the properties of flow field in the vicinity of the cylinder were discussed.