A method of butt-welding, comprising: bringing a solid shaft-shaped end portion of a first workpiece and a solid shaft-shaped end portion of a second workpiece into abutment against each other; wherein, the first workpiece is a cup member constructing an outer joint member of a constant velocity universal joint, and the second workpiece is a shaft member constructing the outer joint member, or the first workpiece is a shaft member constructing an outer joint member of a constant velocity universal joint, and the second workpiece is a cup member constructing the outer joint member, wherein the shaft member has a recess on a radially inner side of a joining end surface, the joining end surface being formed into an annular surface having the inner diameter E, wherein a joining end surface, which is formed on a short small shaft section of the cup member, is annular, and a projecting portion is formed on the radially inner side, and wherein the annular joining end surface has a diameter D on the radially inner side, wherein a portion of the joining end surface of the cup member on the radially inner side protrudes toward the radially inner side with respect to the inner diameter E of the joining end surface of the shaft member and wherein this protruding portion is referred to as a protruding surface, radiating a high energy intensity beam from a radially outer side, wherein dimensions of the protruding surface of the cup member are set so that a relationship of S>=Q is established, where S [S-(E-D)/2] is a width of the protruding surface in a radial direction, and where Q is a height of the back bead from the inner diameter E of the joining end surface of the shaft member, the method further comprising an ultrasonic flow detection step. bringing a solid shaft-shaped end portion of a first workpiece and a solid shaft-shaped end portion of a second workpiece into abutment against each other; wherein, the first workpiece is a cup member constructing an outer joint member of a constant velocity universal joint, and the second workpiece is a shaft member constructing the outer joint member, or the first workpiece is a shaft member constructing an outer joint member of a constant velocity universal joint, and the second workpiece is a cup member constructing the outer joint member, wherein the shaft member has a recess on a radially inner side of a joining end surface, the joining end surface being formed into an annular surface having the inner diameter E, wherein a joining end surface, which is formed on a short small shaft section of the cup member, is annular, and a projecting portion is formed on the radially inner side, and wherein the annular joining end surface has a diameter D on the radially inner side, wherein a portion of the joining end surface of the cup member on the radially inner side protrudes toward the radially inner side with respect to the inner diameter E of the joining end surface of the shaft member and wherein this protruding portion is referred to as a protruding surface, radiating a high energy intensity beam from a radially outer side, wherein dimensions of the protruding surface of the cup member are set so that a relationship of S>=Q is established, where S [S-(E-D)/2] is a width of the protruding surface in a radial direction, and where Q is a height of the back bead from the inner diameter E of the joining end surface of the shaft member, the method further comprising an ultrasonic flow detection step.