Because silicon nitride sintered products are excellent in various properties such as mechanical strength, heat resistance, etc., they are used as high temperature structural materials, such as for parts of gas turbines. However, on the other hand, silicon nitride has a poor sinterability because of having a highly covalent bonding nature, and, consequently, it is difficult to obtain sintered products having a high density and high strength, employing silicon nitride per se. Therefore, in the case of sintering silicon nitride, sintering aids including MgO, Al.sub.2 O.sub.3, oxide of rare earth elements, etc., have been used hitherto. However, sintered products using MgO as a sintering aid have inferior oxidation resistance because MgSiO.sub.3, etc., are formed in the oxidizing atmosphere. In sintered products using Y.sub.2 O.sub.3 as a sintering aid, the oxidation resistance deteriorates extremely if YSiO.sub.2 N and Si.sub.3 Y.sub.2 O.sub.3 N.sub.4 are formed as the second phase (the meaning of which is hereinafter explained) as opposed to forming Si.sub.2 ON.sub.2, Y.sub.2 Si.sub.2 O.sub.7, or Y.sub.5 Si.sub.3 O.sub.12 N. Formation of the second phase is affected by the amount of Si.sub.3 N.sub.4, SiO.sub.2, or Y.sub.2 O.sub.3, the kind and the amount of other sintering aids, the sintering atmosphere, etc., and it is difficult to correctly control these factors. In the case of using CeO.sub.2 as a sintering aid, the second phase composition having good oxidation resistance is not formed in an Si.sub.3 N.sub.4 -CeO.sub.2 system, because CeO.sub.2 is reduced to Ce.sub.2 O.sub.3 during sintering and the produced oxygen reacts with Si.sub.3 N.sub.4 to form SiO.sub.2. In the case of using other oxides of rare earth elements as sintering aids, compositions having improved oxidation resistance have not yet been obtained.