A sealing method is for sealing a gap between an opening section extending through a wall in a thickness direction thereof and an insertion member inserted in the opening section, and includes a seal-member preparing step of preparing a seal member including an elastic layer, an adhesive layer laminated on a surface of the elastic layer, and a release sheet laminated on a surface of the adhesive layer, and formed with a through hole into which the insertion member is to be inserted along a direction of lamination thereof, an inserting step of inserting the insertion member into the through hole such that the insertion member comes in contact with the seal member to bring the release sheet into abutment with the wall, and a sticking step of sticking the seal member to the insertion member and the wall, while peeling the release sheet from the adhesive layer.