A smoothing method and a smoothing apparatus uses a belt grinding apparatus, including an endless grinding belt and a press pad that presses the grinding belt against a welded area, and uses this belt grinding apparatus in two different modes after obtaining information on heights of surfaces of two welded members along a weld bead of the two welded members. The first mode corresponds to a bead removing step in which the grinding belt is pressed against a welded area by the press pad to reduce the height of the weld bead. The next mode corresponds to a smoothing step in which the grinding belt presses against the welded area by the press pad while being made to move along the surfaces of the two welded members to smooth the welded area.