A torsion beam of a coupled torsion beam axle, the torsion beam comprising: an external beam having a first cross-section in which an external protrusion of a middle end portion of the external beam is formed to protrude toward upward in a longitudinal direction of the external beam, and external skirt portions are vertically extended along the longitudinal direction at a first end portion and a second end portion of the external protrusion; and an internal beam having a second cross-section in which an internal protrusion of a middle end portion in the internal beam is inserted into the external beam to face the external protrusion of the external beam, and internal skirt portions are vertically extended at a first end portion and a second end portion of the internal protrusion, and each external surface of the internal skirt portions and each internal surface of the external skirt portions are surface-bonded in a mutually matched state, wherein a gap (g) is formed between an external surface of the internal protrusion and an internal surface of the external protrusion to form a closed cross-section between the external surface of the internal protrusion and the internal surface of the external protrusion, wherein a concave inflection portion is formed at a point where the external surface of the internal protrusion and each external surface of the internal skirt portions are connected, and wherein a direction of bending of the concave inflection portion, starting from the internal skirt portions, changes toward an internal side of the internal beam and thereafter changes toward an external side of the internal beam. an external beam having a first cross-section in which an external protrusion of a middle end portion of the external beam is formed to protrude toward upward in a longitudinal direction of the external beam, and external skirt portions are vertically extended along the longitudinal direction at a first end portion and a second end portion of the external protrusion; and an internal beam having a second cross-section in which an internal protrusion of a middle end portion in the internal beam is inserted into the external beam to face the external protrusion of the external beam, and internal skirt portions are vertically extended at a first end portion and a second end portion of the internal protrusion, and each external surface of the internal skirt portions and each internal surface of the external skirt portions are surface-bonded in a mutually matched state, wherein a gap (g) is formed between an external surface of the internal protrusion and an internal surface of the external protrusion to form a closed cross-section between the external surface of the internal protrusion and the internal surface of the external protrusion, wherein a concave inflection portion is formed at a point where the external surface of the internal protrusion and each external surface of the internal skirt portions are connected, and wherein a direction of bending of the concave inflection portion, starting from the internal skirt portions, changes toward an internal side of the internal beam and thereafter changes toward an external side of the internal beam.