An assembled body attachable to an attached body includes: a main body member made of a rigid material; an annular seal member made of a material that is more easily deformed than the main body member, and assembled on the main body member to close a gap between the main body member and the attached body; and a cover member assembled on the main body member. The main body member is formed with an attachment portion to be disposed inside the attached body, and the cover member is installed on the attachment portion. A seal member attachment region is assigned between a part of the main body member and the cover member, and the seal member attachment region is defined at least by an exposed surface of the attachment portion that is not covered by the cover member and one end surface of the cover member.