material 33 and the material 34 are the same as those shown in the embodiment 1 fig8 is a conceptual view showing a state in which the material 33 the material 34 the cavity portion 38 the cavity portion 39 and the embedded member 40 are set in such a manner as to be welded fig8 shows a positional relation among the butt weld surface 35 the butt weld surface 36 and a rotating tool 37 the rotating tool 37 is inserted from a direction perpendicular to the butt weld surface 35 and the butt weld surface 36 a cavity portion 41 having a circular cross sectional shape is provided by lapping the cavity portion 38 and the cavity portion 39 in such a manner that end portions thereof are aligned with each other the embedded member 40 is set to the cavity portion 41 a cross sectional shape of the embedded member 40 is approximately equal to the cavity portion 41 a radius of the cavity portion 41 formed by the cavity portion 38 having the semicircular cross sectional shape and the cavity portion 39 having the semicircular cross sectional shape is approximately one half of a thickness of the material 33 and is set to the same as described in the embodiment 1 a positional relation between an end surface 43 of the embedded member 40 side in the probe 42 at.