In this paper we study the diffraction of an electromagnetic wave on a high conductivity domain. We observe the formation of a boundary layer in the obstacle and we give a complete asymptotic expansion of this boundary layer. 1. Introduction. We are interested in the propagation of an electromagnetic wave in the vacuum, meeting a conducting domain. In [2], M. Artola and M. Cessenat have studied the diffraction of an electromagnetic wave on a high permittivity and permeability domain. The purpose of this study consists in showing the diffraction of a wave on a high conductivity domain and justifying the perfect conductor's model.