Estimation of directional frequency transfer functions of floating offshore structures is important to estimate their behaviours in realistic short crested irregular waves (such waves are called as directional spectrum waves) Of course, there are some theoretical calculation methods for such directional response functions, but it seems that there are not so many experimental results confirming such theoretical ones. For that purpose, square basins are needed. On the other hand, recently the authors developed a wave maker which can generate directional spectrum waves in a long towing tank, and many experiments of offshore structures are conducted in such waves. So we tried to make inverse estimation of directional transfer functions of a floating offshore structure using both one dimensional response spectrum measumred in such waves, and measured two dimensional directional wave spectrum. Estimated results by present method on heave, pitch, and roll response functions are reported here, but this method will be applicable to other situations like motions of floating structures in actual seas by measuring its motions and incoming waves.