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 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; wherein the cylindrical ring member is engaged on an outer side of the second cylinder body with a gap; wherein the bias mechanism includes a spring; characterized in that the bias mechanism includes a plural number of the springs arranged in line in a circumferential direction of the ring member; the springs being arranged between a bottom face of the ring member and a supporting plate being configured to bias the ring member toward the first 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 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; wherein the cylindrical ring member is engaged on an outer side of the second cylinder body with a gap; wherein the bias mechanism includes a spring; characterized in that the bias mechanism includes a plural number of the springs arranged in line in a circumferential direction of the ring member; the springs being arranged between a bottom face of the ring member and a supporting plate being configured to bias the ring member toward the first cylinder body.