Two-dimensional wave tank tests in regular waves are carried out in order to clarify characteristics and a mechanism of generation of horizontal negative drift force acting on a floating OWC-type wave energy converter “Backward Bent Duct Buoy (BBDB)” in waves. Four kinds of experiments are carried out. The first experiment is a usual drift force testing for the BBDB moored by a soft mooring system. The second one is a drift force testing for the fixed BBDB. The third one is a drift force testing for the BBDB of which the surge motion is only possible. The fourth one is a forced pitch oscillation test in which the BBDB is rotated about the center of the air chamber. From these experiments, the mechanism of the generation of negative drift force acting on BBDB is discussed.