This study aims to make clear the cause and occurrence condition of twodimensional(2-D) freak waves by solving the hydrodynamic equations of 2-D irrotational flow for nonlinear waves with various spectra corresponding to swell from wind waves and describing the long-time evolution. As a result, it is shown that the third order resonant interaction causes the 2-D freak waves of which surface profiles are very similar with those observed in nature and multiplies the occurrence probability with

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... decreasing of the spectral bandwith in deep water. Conversely, the feature of the freak waves -single and outstanding wave height-gets prominent with the broadening of the spectral bandwith.