The present disclosure is characterized by inexpensively treating an ammonia gas contained in an exhaust gas after nitriding without performing combustion, adsorption using an adsorption agent, or the like. A vacuum carburizing device of the present disclosure includes a heating furnace which heats a workpiece, an ammonia gas supply device which supplies an ammonia gas and nitrides the workpiece to the heating furnace, and a thermal decomposition furnace which thermally decomposes the ammonia gas discharged from the heating furnace after nitriding.