The coast in Japan has been often attacked by the typhoon. And then, structures that have been installed there have been hit high waves and strong winds. Especially, for floating structures, safety measures with these high waves and strong winds have been most important subject for their design. In the case of a kind of floating structure positioning by slack mooring system using mooring lines, it has been at risk for breaking of mooring lines by slow drift oscillations contributed non-linear long period fluctuating wave drift forces. Therefore, it has been necessary to evaluate the slow drift oscillation in design process of a floating structure. However, it seems that the evaluation has been not fully carried out, for paucity of the data with slow drift oscillations on open sea test site, and for complexity of calculation method due to non-linearity of slow drift oscillations. So, in this paper, a practical estimation method with slow drift oscillation amplitude has been presented. And this estimation method has been examined for its validity based on the results of open sea tests of an offshore floating wave device“Mighty Whale”. Additionally, coefficients being used in this presented method have been examined.