A grommet comprising: a main body part configured to be fitted to a penetration hole penetrating an attachment target along an axial direction, configured to stop water at the penetration hole, and has a wiring material inserted thereinto along the axial direction; and a tubular part that is unified with the main body part so as to have a tubular shape, and has the wiring material inserted thereinto along the axial direction, wherein the tubular part includes: a tubular part main body that is formed continuing from the main body part; and a fold-back part that is formed continuing from the tubular part main body on a side opposite to a side where the main body part is positioned in the axial direction, and the fold-back part configured to be folded between an attachment position where the fold-back part is folded back to outside of the tubular part main body, and a use position where the fold-back part is positioned on the side opposite to the side where the main body part is positioned with respect to the tubular part main body, characterized in that at the use position, an entire length of an inner surface of the fold-back part in a circumferential direction is longer than an entire length of an inner surface of the tubular part main body in the circumferential direction, and in a state where the fold-back part is folded back toward the tubular part main body, the fold-back part is disposed outside the tubular part main body in a radial direction without causing a contracting force in the radial direction, and the fold-back part includes a folding part that opens to the circumferential direction at the attachment position and that is folded to the circumferential direction at the use position. a main body part configured to be fitted to a penetration hole penetrating an attachment target along an axial direction, configured to stop water at the penetration hole, and has a wiring material inserted thereinto along the axial direction; and a tubular part that is unified with the main body part so as to have a tubular shape, and has the wiring material inserted thereinto along the axial direction, wherein the tubular part includes: a tubular part main body that is formed continuing from the main body part; and a fold-back part that is formed continuing from the tubular part main body on a side opposite to a side where the main body part is positioned in the axial direction, and the fold-back part configured to be folded between an attachment position where the fold-back part is folded back to outside of the tubular part main body, and a use position where the fold-back part is positioned on the side opposite to the side where the main body part is positioned with respect to the tubular part main body, characterized in that at the use position, an entire length of an inner surface of the fold-back part in a circumferential direction is longer than an entire length of an inner surface of the tubular part main body in the circumferential direction, and in a state where the fold-back part is folded back toward the tubular part main body, the fold-back part is disposed outside the tubular part main body in a radial direction without causing a contracting force in the radial direction, and the fold-back part includes a folding part that opens to the circumferential direction at the attachment position and that is folded to the circumferential direction at the use position. a tubular part main body that is formed continuing from the main body part; and a fold-back part that is formed continuing from the tubular part main body on a side opposite to a side where the main body part is positioned in the axial direction, and the fold-back part configured to be folded between an attachment position where the fold-back part is folded back to outside of the tubular part main body, and a use position where the fold-back part is positioned on the side opposite to the side where the main body part is positioned with respect to the tubular part main body, characterized in that at the use position, an entire length of an inner surface of the fold-back part in a circumferential direction is longer than an entire length of an inner surface of the tubular part main body in the circumferential direction, and in a state where the fold-back part is folded back toward the tubular part main body, the fold-back part is disposed outside the tubular part main body in a radial direction without causing a contracting force in the radial direction, and the fold-back part includes a folding part that opens to the circumferential direction at the attachment position and that is folded to the circumferential direction at the use position.