Combination of a coupling for coupling tubes to each other or a tube with another device, together with at least one of such tubes, the coupling comprising: a fitting portion which is formed at one axial end of the coupling and on which the tube is fitted; a coupling portion which is formed at the other axial end of the coupling and is adapted to be coupled with a counterpart coupling; an engagement hook which is provided in the coupling portion and is adapted to be engaged with an engagement hook of the counterpart coupling; a sealing face which is adapted to axially abut and close a gap with a sealing face of the counterpart coupling; a fluid passage which is formed inside the coupling; and an engagement face which is provided at the engagement hook of the coupling and is inclined at a predetermined angle in an overhanging manner with respect to a circumferential direction of the coupling, the engagement face which makes the engagement hook engage further tightly with the engagement hook of the counterpart coupling causing an axial load, which acts in a direction to separate the couplings from each other in an engagement state where the engagement hook is engaged with each other, said axial load generating a component force in the circumferential direction of the coupling according to the predetermined angle and bringing the engagement hooks further closer to each other in the circumferential direction in the engagement state, characterized in that the engagement face keeps the predetermined angle in a range where the engagement face is still inclined in the overhanging manner at a positive angle with respect to the circumferential direction of the coupling and the engagement hooks are still maintained in the engagement state, even if the coupling portions or the couplings are deformed when an excess load is applied axially to the coupling portions or the couplings during pressure of a fluid unusually increases up to bursting pressure of the tube. a fitting portion which is formed at one axial end of the coupling and on which the tube is fitted; a coupling portion which is formed at the other axial end of the coupling and is adapted to be coupled with a counterpart coupling; an engagement hook which is provided in the coupling portion and is adapted to be engaged with an engagement hook of the counterpart coupling; a sealing face which is adapted to axially abut and close a gap with a sealing face of the counterpart coupling; a fluid passage which is formed inside the coupling; and an engagement face which is provided at the engagement hook of the coupling and is inclined at a predetermined angle in an overhanging manner with respect to a circumferential direction of the coupling, the engagement face which makes the engagement hook engage further tightly with the engagement hook of the counterpart coupling causing an axial load, which acts in a direction to separate the couplings from each other in an engagement state where the engagement hook is engaged with each other, said axial load generating a component force in the circumferential direction of the coupling according to the predetermined angle and bringing the engagement hooks further closer to each other in the circumferential direction in the engagement state, characterized in that the engagement face keeps the predetermined angle in a range where the engagement face is still inclined in the overhanging manner at a positive angle with respect to the circumferential direction of the coupling and the engagement hooks are still maintained in the engagement state, even if the coupling portions or the couplings are deformed when an excess load is applied axially to the coupling portions or the couplings during pressure of a fluid unusually increases up to bursting pressure of the tube.