A deforming apparatus (10) for a steel wire (W) for a bead core (X) includes a deforming roller (12), a guide device (20), and a deforming amount changing device (30). The deforming roller (12) is arranged upstream of a molding drum (50), which molds the bead core (X) by winding the steel wire (W). The steel wire (W) is mounted on the deforming roller (12) in a curved manner such that the deforming roller (12) deforms the steel wire (W). The guide device (20) is arranged upstream of the deforming roller (12) and guides the steel wire (W) to the deforming roller (12). The deforming amount changing device (30) changes the amount by which the steel wire (W) is mounted on the deforming roller (12) in a curved manner by moving the guide device (20) with respect to the deforming roller (12).