PURPOSE: To smoothen the surface of a fired molded ceramic substrate by coating the surface of the substrate with a fine powdery ceramic forming material and firing the substrate again.

CONSTITUTION: A ceramic forming material 2 of 0.3-1.5μm particle size is mixed with an aq. soln. of a binder such as methylcellulose to prepare a mixed fluid 4. The surface of a fired molded ceramic substrate 1 with pores 3 of 10-50μm diameter in the surface is coated with the fluid 4 to fill the pores 3 and the coated substrate 1 is dried and fired at about 300°C. Powder 6 on the surface of the substrate 1 is then removed with a scraper 5 and the substrate 1 is fired again at about 1,500°C to sinter powder in the pores 3. The pores 3 are blocked up with ceramic sintered bodies 7 and a poreless ceramic substrate A is obtd.