A first joint portion (C1, C1') which is capable to be releasably connected with a second joint portion (C2, C2') in order to constitute a hydraulic joint structure (C, C'), wherein: the first joint portion (C1, C1') has a first oil passage and a second oil passage; the first joint portion (C1, C1') has a cylindrical male member; the first oil passage (114; 114') of the first joint portion (C1, C1') extends in the male member of the first joint portion (C1, C1') and has a first open end portion opening into an outer circumference of the male member at a first axial position of the male member; the second oil passage (116; 116') of the first joint portion (C1, C1') extends in the male member of the first joint portion (C1, C1') and has a second open end portion opening into an outer circumference of the male member at a second axial position of the male member, said first joint portion (C1, C1') further comprising a ring member, which has a cylindrical hole into which the male member of the first joint portion (C1, C1') is inserted and which is movable between an oil passage-covering position and a retracted position along an axial direction of the male member, wherein: the ring member is biased toward the oil passage-covering position by a spring; the ring member has an inner circumference provided therein with a communication groove; and when the ring member is positioned at the oil passage-covering position, the first open end portion of the first oil passage of the male member and the second open end portion of the second oil passage of the male member are connected via the communication groove. the first joint portion (C1, C1') has a first oil passage and a second oil passage; the first joint portion (C1, C1') has a cylindrical male member; the first oil passage (114; 114') of the first joint portion (C1, C1') extends in the male member of the first joint portion (C1, C1') and has a first open end portion opening into an outer circumference of the male member at a first axial position of the male member; the second oil passage (116; 116') of the first joint portion (C1, C1') extends in the male member of the first joint portion (C1, C1') and has a second open end portion opening into an outer circumference of the male member at a second axial position of the male member, said first joint portion (C1, C1') further comprising a ring member, which has a cylindrical hole into which the male member of the first joint portion (C1, C1') is inserted and which is movable between an oil passage-covering position and a retracted position along an axial direction of the male member, wherein: the ring member is biased toward the oil passage-covering position by a spring; the ring member has an inner circumference provided therein with a communication groove; and when the ring member is positioned at the oil passage-covering position, the first open end portion of the first oil passage of the male member and the second open end portion of the second oil passage of the male member are connected via the communication groove. the ring member is biased toward the oil passage-covering position by a spring; the ring member has an inner circumference provided therein with a communication groove; and when the ring member is positioned at the oil passage-covering position, the first open end portion of the first oil passage of the male member and the second open end portion of the second oil passage of the male member are connected via the communication groove.