We propose a method to estimate the surface normal of concave objects. The target object of our method has a specular surface without diffuse reflection. We solve the problem by analyzing the polarization state of the reflected light. The polarization analysis gives a constraint to the surface normal. However, polarization data from a single view has an ambiguity and cannot uniquely determine the surface normal. To solve this problem, the target object should be observed from two or more views. However, the polarization of the light should be analyzed at the same surface point through the different views. This means that both the camera parameters and the surface shape should be known. The camera parameters can be estimated a priori using known corresponding points. However, it is a contradiction that the shape should be known in order to estimate the shape. To resolve this problem, we assume that the target object is almost planar. Under this assumption, the surface normal of the object is uniquely determined. We show that the surface normal of the nonplanar part can also be estimated using the proposed method.