Provided is a method for welding an accessory part made of a thermoplastic resin to a parison which is being molded into a tank body. The method includes steps of: forming the accessory part to include an annular portion having an annular shape and a plurality of projections each projecting from the annual portion in a radially inward direction; and pressing the annular portion and the projections into the parison to be welded to the parison. Also provided is a welding structure of a tank body made of a resin and an accessory part made of a thermoplastic resin and welded to a wall surface of the tank body. The accessory part includes: an annular portion having an annular shape; and a plurality of projections each projecting from the annular portion in a radially inward direction. The annular portion and the plurality of projections are welded to the wall surface.