A hermetically sealing device for being attached to a bored opening bored through a side of a fluid pipe, the hermetically sealing device comprising: a sealing main-body; and a rubber seal adhered to the sealing main-body, wherein the rubber seal includes: an inner lip for extending along a periphery of the bored opening, and that includes an outer-circumference surface that extends forward and inclines toward an inner-circumference side; a slit that is formed on an outer-circumference side relative to the inner lip and where an outer-surface-side edge of the bored opening can enter; and an outer lip that is formed on an outer-circumference side relative to the slit that is formed on an outer-circumference side relative to the inner lip, and transforms in such a manner that the outer lip extends toward an outer-circumference side for being hermetically in contact with an outer surface of the periphery of the bored opening; characterised in that the rubber seal includes rear rubber that is disposed behind the outer lip and that is made of rubber that is harder than the outer lip a sealing main-body; and a rubber seal adhered to the sealing main-body, wherein the rubber seal includes: an inner lip for extending along a periphery of the bored opening, and that includes an outer-circumference surface that extends forward and inclines toward an inner-circumference side; a slit that is formed on an outer-circumference side relative to the inner lip and where an outer-surface-side edge of the bored opening can enter; and an outer lip that is formed on an outer-circumference side relative to the slit that is formed on an outer-circumference side relative to the inner lip, and transforms in such a manner that the outer lip extends toward an outer-circumference side for being hermetically in contact with an outer surface of the periphery of the bored opening; characterised in that the rubber seal includes rear rubber that is disposed behind the outer lip and that is made of rubber that is harder than the outer lip an inner lip for extending along a periphery of the bored opening, and that includes an outer-circumference surface that extends forward and inclines toward an inner-circumference side; a slit that is formed on an outer-circumference side relative to the inner lip and where an outer-surface-side edge of the bored opening can enter; and an outer lip that is formed on an outer-circumference side relative to the slit that is formed on an outer-circumference side relative to the inner lip, and transforms in such a manner that the outer lip extends toward an outer-circumference side for being hermetically in contact with an outer surface of the periphery of the bored opening; characterised in that the rubber seal includes rear rubber that is disposed behind the outer lip and that is made of rubber that is harder than the outer lip a slit that is formed on an outer-circumference side relative to the inner lip and where an outer-surface-side edge of the bored opening can enter; and an outer lip that is formed on an outer-circumference side relative to the slit that is formed on an outer-circumference side relative to the inner lip, and transforms in such a manner that the outer lip extends toward an outer-circumference side for being hermetically in contact with an outer surface of the periphery of the bored opening; characterised in that the rubber seal includes rear rubber that is disposed behind the outer lip and that is made of rubber that is harder than the outer lip