At least one underfill part is formed so that a central axis of the arm component coincides with a load line of the arm component. The central axis of the configuration is a line connecting centroids of cross-sectional shapes with one another in an axial direction of the arm component, the cross-sectional shapes being shapes on cross-sectional planes perpendicular to an axis of the arm component. The load line of the arm component is a line on which a load is imposed in the arm component.