Strain and stress fields, which accompany the diffusion flux through an infinite thin plate, are studied. A complex tension is realized in a plate despite an admission, and is developed in the course of time. An essential inhomogeneity and instationarity of deformation results in the appearance of additional elastic-viscous stresses depending on a strain velocity. The shear components of elastic-viscous stresses have large values. These values are able to change the present structure.