Model tests of a semi-submersible have been conducted in directional spectra waves generated by the new snake type wave maker installed in the towing tank of Yokohama National University. In various kinds of short-crested irregular waves which have specified power spectra and directional distribution functions, motion responses of the moored semi-submersible ITTC model were measured. The 1st-order motion responses in wave frequency range were estimated by the linear superposition theory using directional wave spectra and transfer functions in oblique waves. Successful agreement between measured and estimated responses was obtained even in wide spreading waves as well as in narrow spreading waves. The influence of directional spreading function and principal wave direction on the short-term prediction of motions was also investigated and some examples are presented.