According to the present invention, a side part cavity (23) in which a side part is molded, a body part cavity (24) in which a body part is molded, and an attachment part cavity (25) in which an attachment part is molded are formed between a plurality of molding molds (21, 22), and a guiding part (26) that is configured to guide a foam to the body part cavity when an undiluted foam solution of the second synthetic resin material supplied to the side part cavity foams is arranged at a part of the side part cavity adjacent to, in a left-right direction X, a rear end part of the body part cavity in which a non-contact part is molded.