When a plurality of outer rings (R) are sequentially and inductively heated to a target temperature by energizing a heating coil (2) elongated more than each of the plurality of outer rings (R), which are made of steel and retained coaxially by a retaining unit (3), while relatively moving the outer rings (R) and the heating coil (2) in an axial direction, an output of the heating coil (2) is varied in consideration of the number of outer rings (R) present in an opposing region of the heating coil (2).