A combustion synthesis method for the synthesis of α-Si3N4 from a reactant compact composed of Si, NaN3, and NH4X and wrapped up with an igniting agent was investigated. Wrapping the reactant compact with the igniting agent (i.e., a mixture of Ti and C powders) was found necessary for the synthesis of Si3N4. In addition to NH4Cl, which was considered previously to function as a catalytic agent, other ammonium halides (i.e., NH4F, NH4Br, and NH4I) were found to be capable of catalyzing the synthesis reaction with NH4Cl being the most effective. Si3N4 could not be produced when NaN3 was replaced by C3H6N6. NaN3 was thus considered to exert an essential effect on the combustion synthesis reaction other than functioning as a solid-state nitrogen source as considered previously. It was proposed that Na vapor produced by decomposition of NaN3 reduces SiXx (formed by reaction of Si and NH4X), promoting the nitridation reaction to form Si3N4. NaN3 thus plays a role as a reducing agent in the synthesis reaction.