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 (R) 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 (R), 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 (R) 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 (R), 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 . <EOS>