The present invention relates to a cotter-mounting apparatus that, by engaging a pair of cotters with engagement grooves provided on the outer circumferential surface of an axial member, mounts the pair of cotters that fix a retainer for holding a spring to exert a pressing force to the axial member in a compressed state, wherein the cotter-mounting apparatus includes a retainer holding mechanism that holds a retainer having a pair of cotters incorporated, and is driven to be relatively elevated and lowered on the upper side in the axial line direction of the axial member; a cotter separating mechanism for separating the pair of cotters from each other over the outer diameter of the axial member roughly in the horizontal direction while pushing up the pair of cotters from downward and for holding the pair of cotters; and a cotter pressing mechanism for pressing the pair of cotters separated by the cotter separating mechanism from the outside roughly in the horizontal direction and causing the cotters to be engaged with the engagement grooves positioned in advance. With such a structure, it is possible to mount cotters with a simple procedure, to increase productivity and to engage the pair of cotters with a plurality of engagement grooves without fail.