In 1986, Masuda proposed a floating wave energy converter with oscillating water column and it was named Backward Bent Duct Buoy (BBDB). In order to design a floating OWC-type wave energy converter such as BBDB optimally, it is necessary to develop a numer- ical analysis code in time domain on motions of BBDB, air flow in air chamber and rotation of air turbine in waves. As a first step for developing such numerical code, in this paper, first, equations of motion of a floating OWC-type wave energy converter in waves considering memory effect by air pressure in air chamber are shown. Second, some calculation results in frequency domain by 3D boundary element method, which are a base of above calculation in time domain, are shown. Experiments on exciting forces and radiation forces are also carried out and calculation results are compared with experimental results in frequency domain. In addition, BBDB has neg- ative drift force in specific frequency region and it is im- portant to estimate mooring cost for practical use. How- ever, detail on it is still not clear. So, to make it clear experimentally, four kinds of tank tests were carried out and discussed in this research.