A first nitriding process step is performed in which a steel member is subjected to a nitriding process in a nitriding gas atmosphere having a nitriding potential with which a nitride compound layer having a γ′ phase or an e phase is generated, and thereafter a second nitriding process step is performed in which the steel member is subjected to a nitriding process in a nitriding gas atmosphere having a nitriding potential lower than the nitriding potential in the first nitriding process step, to thereby precipitate the γ′ phase in the nitride compound layer. It is possible to generate the nitride compound layer having a desired phase mode uniformly all over a component to be treated and to manufacture a nitrided steel member high in pitting resistance and bending fatigue strength.