A composite ceramic is produced by forming a mixed powder of Si and a compound oxide as the starting material into a formed material, nitriding and firing the formed material to form a sinter, and subjecting the sinter to oxidation treatment. The composite ceramic, which is:lightweight and reduced in heat conductivity, has a specific gravity of at most 2.6, and comprises a phase composed of all of elements Si, O and N, and at least one element selected from the element group consisting of A1, Zr, Li, P and Mg, in which phase a subphase composed of Si, Al, 0 and N and a subphase composed of Si, O and N are formed. Another composite ceramic has pores and a disperse phase in a matrix phase composed of Si.sub.3 N.sub.4 crystal grains, an oxide and an oxynitride, and further comprises coarse Si.sub.3 N.sub.4 grains dispersed in the matrix phase with O-containing boundary reaction product layers formed in the boundaries between the matrix phase and the coarse Si.sub.3 N.sub.4 grains. Thus, this composite ceramic, though reduced in heat conductivity, has a high toughness and a high strength due to dispersion of the coarse Si.sub.3 N.sub.4 grains in the matrix phase.