We placed a Ni-metal foil between two MgO blocks, and annealed this set in air. The Ni-metal foil entirely reacted with gaseous oxygen to NiO. Thus, we accomplished the joining of two MgO blocks to form MgO-NiO solid solution at the joining part. This technique is highly suitable to weld ceramics into one body with a solely continuous interface. It will be also of great help in forming ceramic products into a complicated or very long shape. We examined the joining, using a nickel foil 10 or 50 pm thick, at 1573-1873K for 176.4-1440ks. The joining layer consisted of the functionally gradient material of MgO-NiO system. The fracture strength of joints annealed at 1573K was very low value, but, those of the joints annealed at 1873K ranged from 40MPa to 95MPa and average value reached about 70% of the strength of the MgO samples. About the penetration curves of the MgO-NiO system, assuming quasi-binary system, and by plane source method, we could obtain the chemical diffusion coefficient, D of 3.3x10^-15m²s^-1 at 1573K.