A ceramic heater 1 includes a silicon nitride ceramic substrate 13 and a resistance-heating member 10 embedded in the silicon nitride ceramic substrate 13 . The silicon nitride ceramic substrate 13 contains oxygen derived from a sintering aid at an average concentration of 0.4 3.2% by weight in a surface layer portion extending from the surface thereof to a depth of 1 mm. The ceramic heater 1 can be manufactured by firing through hot pressing. A firing jig for use in a firing process has an SiC--C composite layer which is formed therein along the surface of cavities thereof while having a predetermined depth as measured from the cavity surface.