Using those results which were judged to be reasonable among various experiments, an equation predicting the threshold shear velocity on a wet sand surface was obtained. Then, based on a literature survey, results from fundamental experiments, and information obtained from a series of field observation carried out by the authors, a hypothesis to explain the blown sand phenomena on a wet sand surface was developed. Experiments with a well-sorted sand having a median diameter of 0.3 mm showed that the prediction was valid if the water content of the sand layer was less than 8 %.