Local scour is one of the most important causes on bridge failures during floods, and recent changes of river environments makes it difficult to predict the scour depth and design an appropriate protection works. In this study, firstly an analytical model to describe scouring process around a pier has been derived based on reasonable understanding of its essential mechanism . And then, manipulating this model, the similarity law to govern a distorted model test has been established, and furthermore, the subsidiary effects which cannot be reproduced even in a distorted model has been reasonably evaluated.