Using a conveyance roller having an outer peripheral surface which is capable of conveying two sheets with an elastic element sandwiched therebetween, in a longitudinal direction of the sheets, and formed with a convex section; and a clamping and pressing device, the sheets with the elastic element sandwiched therebetween are ultrasonic-welded together under pressing. The convex section is formed in a shape extending along a line intersecting a conveyance direction of the conveyance roller, and the convex section is provided with a plurality of grooves at positions spaced apart from each other in a longitudinal direction thereof, wherein each of the grooves extends in the conveyance direction of the conveyance roller to allow a portion of the sheets on which the elastic element is disposed, to be inserted therein.