Abstract: A method of production of a radio frequency accelerator which has a tubular part 1 which forms an acceleration cavity, including a temporary assembly step of making a plurality of component members 11 to 14 which have shapes obtained by splitting the tubular part 1 mate with each other to temporarily assemble them into the shape of the tubular part 10 and a welding step of welding the plurality of component members 11 to 14 together. The temporary assembly step includes a step of placing, inside of the tubular part 1, support members 21 for contacting the inside surface of the tubular part 1 and supporting the tubular part 1 from the inside, and the welding step includes a step of welding the plurality of component members 11 to 14 along the butt lines 51 by friction stir welding.