A jack device attached to a machine body of a work machine including the machine body to be able to lift up the machine body, the jack device comprising: an arm mountable on the machine body so as to be rotatable about an arm rotation axis extending in a machine up-down direction, which is an up-down direction of the machine body; a jack body including a jack cylinder expandable and contractable in a cylinder expansion-contraction direction and attached to the arm rotatably about a cylinder rotation axis to be shiftable between an upright posture and a storage posture, the upright posture being a posture where the jack body is capable of lifting up the machine body by expansion of the jack cylinder and the storage posture being a posture where the cylinder expansion-contraction direction is tilted from the machine up-down direction more largely than the upright posture and the jack cylinder lies along an upper surface of the arm, characterised in that a reaction-force application unit is supported by the arm and configured to apply a reaction force to the jack body in response to a force applied from the jack body to make a reaction-force moment (Mr) act on the jack body in all postures from the upright posture to the storage posture, the reaction-force moment being a moment caused by the reaction force about the cylinder rotation axis and having a direction to make the jack body closer to the storage posture, wherein the reaction-force application unit is configured to make the reaction-force moment greater than a self-weight moment (Ms) act on the jack body in the storage posture and configured to make the reaction-force moment smaller than the self-weight moment act on the jack body in the upright posture, the self-weight moment being a moment caused by a weight of the jack body about the cylinder rotation axis and having a direction to make the jack body closer to the upright posture. an arm mountable on the machine body so as to be rotatable about an arm rotation axis extending in a machine up-down direction, which is an up-down direction of the machine body; a jack body including a jack cylinder expandable and contractable in a cylinder expansion-contraction direction and attached to the arm rotatably about a cylinder rotation axis to be shiftable between an upright posture and a storage posture, the upright posture being a posture where the jack body is capable of lifting up the machine body by expansion of the jack cylinder and the storage posture being a posture where the cylinder expansion-contraction direction is tilted from the machine up-down direction more largely than the upright posture and the jack cylinder lies along an upper surface of the arm, characterised in that a reaction-force application unit is supported by the arm and configured to apply a reaction force to the jack body in response to a force applied from the jack body to make a reaction-force moment (Mr) act on the jack body in all postures from the upright posture to the storage posture, the reaction-force moment being a moment caused by the reaction force about the cylinder rotation axis and having a direction to make the jack body closer to the storage posture, wherein the reaction-force application unit is configured to make the reaction-force moment greater than a self-weight moment (Ms) act on the jack body in the storage posture and configured to make the reaction-force moment smaller than the self-weight moment act on the jack body in the upright posture, the self-weight moment being a moment caused by a weight of the jack body about the cylinder rotation axis and having a direction to make the jack body closer to the upright posture.