A method for manufacturing a head stack assembly that includes a pair of suspensions each of which supports a head, an arm that drives the head, and a pair of base plates that attach the pair of suspensions to both sides of the arm that has a perforation hole, each of the pair of base plates having a boss with an opening includes the step of swaging the pair of base plates with the arm by passing, in one direction, a processing member larger than the opening through the perforation hole and the base plates while the pair of base plates are being inserted into the perforation hole at both sides of the perforation hole, wherein where a neutral plane is defined as a plane that halves a thickness of the arm and is perpendicular to a section that includes a center axis of the perforation hole, a first position is defined as a position that has a minimum internal diameter of the base plate, and a second position is defined as a position closest to the neutral plane among contact positions between the base plates and the arm, a distance between the neutral plane and the first position is equal to or smaller than a distance between the neutral plane and the second position before the swaging step.