A sealing structure, comprising: a filling chamber structure having a filling chamber body with a filling chamber for filling fluid in the filling chamber and a cylindrical body formed in a cylindrical shape integrally with the filling chamber body; a valve body having an insertion portion with a cylindrical outer shape inserted in the cylindrical body; an O-ring provided between the Cylindrical body and the insertion portion to seal fluid; and a first backup ring that is endless and disposed on one side, with respect to an axial direction, of the O-ring to restrict movement of the O-ring along the axial direction, wherein an annular housing groove is formed on an inner circumferential surface of the cylindrical body such as to be recessed outward with respect to a radial direction in order to house the O-ring and the first backup ring, wherein the cylindrical body is provided with a first cylindrical portion integral with the filling chamber body and a second cylindrical portion that is attachable/detachable along the axial direction to/from the first Cylindrical portion, characterized in that the housing groove is divided along the axial direction at a dividing plane into a first half groove and a second half groove, the first half groove being formed on the first cylindrical portion and the second half groove being formed on the second cylindrical portion, such that the first half grove. faces outward when the second cylindrical portion detaches from the first cylindrical portion, wherein the bottom surface of the housing groove includes a first bottom surface that is parallel to the axial direction, a second bottom surface (21 b) that is continuous, on the inner side with respect to the axial direction, with the first bottom surface, and a third bottom surface (21 c) that is continuous, on the outer side with respect to the axial direction, with the first bottom surface (21 a), wherein the second and third bottom surfaces (21b, 21 c) are tapered surfaces inclined such that the depth of the housing groove becomes larger toward the first bottom surface (21 a), and wherein the dividing plane is disposed at the border position, with respect to the axial direction, between the first bottom surface (21 a) and the third bottom surface. a filling chamber structure having a filling chamber body with a filling chamber for filling fluid in the filling chamber and a cylindrical body formed in a cylindrical shape integrally with the filling chamber body; a valve body having an insertion portion with a cylindrical outer shape inserted in the cylindrical body; an O-ring provided between the Cylindrical body and the insertion portion to seal fluid; and a first backup ring that is endless and disposed on one side, with respect to an axial direction, of the O-ring to restrict movement of the O-ring along the axial direction, wherein an annular housing groove is formed on an inner circumferential surface of the cylindrical body such as to be recessed outward with respect to a radial direction in order to house the O-ring and the first backup ring, wherein the cylindrical body is provided with a first cylindrical portion integral with the filling chamber body and a second cylindrical portion that is attachable/detachable along the axial direction to/from the first Cylindrical portion, characterized in that the housing groove is divided along the axial direction at a dividing plane into a first half groove and a second half groove, the first half groove being formed on the first cylindrical portion and the second half groove being formed on the second cylindrical portion, such that the first half grove. faces outward when the second cylindrical portion detaches from the first cylindrical portion, wherein the bottom surface of the housing groove includes a first bottom surface that is parallel to the axial direction, a second bottom surface (21 b) that is continuous, on the inner side with respect to the axial direction, with the first bottom surface, and a third bottom surface (21 c) that is continuous, on the outer side with respect to the axial direction, with the first bottom surface (21 a), wherein the second and third bottom surfaces (21b, 21 c) are tapered surfaces inclined such that the depth of the housing groove becomes larger toward the first bottom surface (21 a), and wherein the dividing plane is disposed at the border position, with respect to the axial direction, between the first bottom surface (21 a) and the third bottom surface.