A coupling device (9, 10) with a first coupler (73, 83) and a second coupler (74, 84), the coupling device (9, 10) configured to connect the first coupler (73, 83) and the second coupler (74, 84) by moving one coupler (74, 84) of the first coupler (73, 83) and the second coupler (74, 84) to the other coupler (73, 83) of the first coupler (73, 83) and the second coupler (74, 84), the first coupler (73, 83) including a first cylinder body, the second coupler (74, 84) including a second cylinder body facing the first cylinder body, a cylindrical ring member coaxial with the second cylinder body, disposed outside the second cylinder body and movable in an axial direction of the second cylinder body, a seal ring arranged between the second cylinder body and the ring member, and a bias mechanism biasing the ring member toward the first cylinder body, a sealing member being disposed on at least one of an end face of the first cylinder body and an end face of the ring member, and when the one coupler (74, 84) is moved toward the other coupler (73, 83), the end face of the first cylinder body contacting the end face of the ring member via the sealing member, whereby an inner passage of the first cylinder body is in communication with an inner passage of the second cylinder body, wherein the coupling device (9, 10) further comprises a retainer provided on the second coupler (74, 84) and configured to prevent removal of the ring member from the second cylinder body; characterized in that the cylindrical ring member is engaged on an outer side of the second cylinder body with a gap; wherein an annular groove is formed in an inner circumferential surface of the ring member; wherein the ring member has an inner diameter uniform except the annular groove and the inner diameter of the ring member is larger than an outer diameter of the second cylinder body. the first coupler (73, 83) including a first cylinder body, the second coupler (74, 84) including a second cylinder body facing the first cylinder body, a cylindrical ring member coaxial with the second cylinder body, disposed outside the second cylinder body and movable in an axial direction of the second cylinder body, a seal ring arranged between the second cylinder body and the ring member, and a bias mechanism biasing the ring member toward the first cylinder body, a sealing member being disposed on at least one of an end face of the first cylinder body and an end face of the ring member, and when the one coupler (74, 84) is moved toward the other coupler (73, 83), the end face of the first cylinder body contacting the end face of the ring member via the sealing member, whereby an inner passage of the first cylinder body is in communication with an inner passage of the second cylinder body, wherein the coupling device (9, 10) further comprises a retainer provided on the second coupler (74, 84) and configured to prevent removal of the ring member from the second cylinder body; characterized in that the cylindrical ring member is engaged on an outer side of the second cylinder body with a gap; wherein an annular groove is formed in an inner circumferential surface of the ring member; wherein the ring member has an inner diameter uniform except the annular groove and the inner diameter of the ring member is larger than an outer diameter of the second cylinder body.