In order to apply one-dimensional elasto-viscous theory to practical problems, it is necessary to extend it to three-dimensional theory. With this in mind, three-dimensional creep tests under drained conditions were carried out on a reconstituted alluvial clay in a triaxial cell. A question of whether volumetric and shear viscous coefficients are independent of stress conditions or not, was examined. The result revealed that volumetric and shear viscous coefficients of clay are independent of stress condition. Furthermore, a unique plane termed ‘viscous coefficient plane’ exists in a space of e, logσ' and logη.