A method of manufacturing an electrochemical device saving an electrochemical cell including a planar solid elec- rolyte body (2, 4) and at least two electrodes (10, 12) disposed in contact with the solid electrolyte body. So that the lectrochemical device does not suffer from warpage that vould arise due to difference in firing shrinkage between the layers, there is formed a first unfired ceramic layer (18) on one side of the unfired structure of the electrochemical cell 2,4,6), the first unfired ceramic layer being given a porous tructure by firing thereof, and a second unfired ceramic layer 20) on the other side of the unfired structure of the electro- hemical cell, the second unfired ceramic layer having sub- tantially the same property of firing shrinkage as the first nfired ceramic layer. The unfired structure of the electro- hemical cell, the first and second unfired ceramic layers is o-fired. The two unfired ceramic layers may be formed on pposite sides of an unfired structure of an electrochemical ell assembly which includes plural cells, for example, an xygen pumping cell and an oxygen sensing cell. In this case, nother unfired ceramic layer may be formed between the nfired structures of the two cells.