In the eld of computer vision, bene cial methods of measuring surface shape of transparent objects such as glasses have rarely been proposed. In this paper, we propose a convenient and inexpensive method for measuring the surface shape of transparent objects. The degree of polarization of the light re ected from the object surface depends on the re ection angle which, in turn, depends on the object's surface normal thus, by measuring the degree of polarization, we are able to calculatethe surface normalofthe object. But unfortunately, the relationship between degree of polarization andsurface normalisnot1to 1 thus, to obtainthe true surface normal,we have to resolve this ambiguityproblem. In this paper, we explain the method of resolving the ambiguity by using the di erential-geometrical property of the object surface.