The at-sea experiment of the moored floating platform “POSEIDON” has been carried out at Japan Sea since 1986 for four years. In this experiment, three ultra-sonic type wave probes were installed as a line array at the sea bottom of the test area, and the directional wave spectra were obtained using measured three time series of water surface elevation by Maximum Likelihood Method (MLM). Also, the six components of motions of POSEIDON were measured by accelerometer and vertical/directional gyro. In this paper, the authors tried to estimate the directional frequency response functions in wave frequency range using these field data. The results estimated by present method were compared with the theoretical response functions and it was found that we need many experimental runs of various wave directions for obtaining fairly good response functions.