To provide a method for processing an iron-based alloy, which is simpler than a conventional surface treating method and by which a member having excellent properties such as fatigue strength can be obtained.

This method for processing the iron-based alloy comprises a plasticizing process including strong processing for imparting strain (=) corresponding to ≥4 to at least a part of a member formed from the iron-based alloy and a surface treating process for making an element to diffuse and intrude into the obtained strongly processed member from the surface. It is estimated that the intrusion and diffusion properties of the element at the time of the surface treatment such as nitridation treatment can be improved by carrying out the prescribed strong processing as described above before conducting the surface treatment such as nitridation treatment. That is to say, the fining of crystal particles and the formation of areas wherein dislocations accumulate in crystal are enhanced by imparting the prescribed strain to the member, and a large amount of the element intruding into the crystal grain boundaries of fine crystal particles or dislocation parts is trapped, and thereby the diffusion of the element intruding into the inside of the member is accelerated.