A swing arm of a suspension system for supporting the body of a motor vehicle comprises a main swing arm body on which a support portion for supporting a suspension damper is provided, a support member fixed to a rear portion of the main swing arm body and having supporting parts for supporting first to third suspension arms, and a support reinforcing member for reinforcing at least the supporting parts of the support member for supporting the second and third suspension arms. The main swing arm body, the support member and the support reinforcing member are produced by individually press-forming single steel plates and the swing arm is formed by joining the main swing arm body, the support member and the support reinforcing member.