A landing gear comprising: a pair of skids configured to be arranged in parallel with a front-rear axis of an airframe of a rotary wing aircraft; a cross tube configured to be attached to the airframe and coupling the pair of skids to each other, the cross tube including an internal space; and a stiffening portion configured to suppress flattening of the cross tube 30), wherein: the cross tube includes curved pipe parts located closer to corresponding end portions of the cross tube than corresponding attached portions of the cross tube, the attached portions being configured to be attached to the airframe; characterized in that the stiffening portion is arranged in at least one of internal spaces of pipe parts (37, 32) that are the curved pipe parts and an internal space of a pipe part located between the pair of curved pipe parts and said stiffening portion includes an enlarged diameter portion (41, 42, 43) and a fastening portion (60, 61, 64), wherein said enlarged diameter portion (41, 42, 43) is configured to increase in diameter by axial fastening power of the fastening portion (60, 61, 64) and to be brought into contact with an inner surface of the cross tube, and said fastening portion (60, 61, 64) is configured to generate said axial fastening power with respect to said enlarged diameter portion (41, 42, 43), the stiffening portion includes a ring capable of increasing in diameter and configured to increase in diameter to be brought into contact with an inner surface of the pipe part (37, 32), a first disc and second disc internally fitted to the ring and configured to sandwich the ring in an axial direction, and a wire configured to generate axial fastening power with respect to the first disc and the second disc; the first disc includes a first engaging portion which is brought into contact with the ring to restrict the displacement of the ring toward one side in the axial direction and increase the diameter of the ring; and the second disc includes a second engaging portion which is brought into contact with the ring to restrict the displacement of the ring toward the other side in the axial direction and increase the diameter of the ring. a pair of skids configured to be arranged in parallel with a front-rear axis of an airframe of a rotary wing aircraft; a cross tube configured to be attached to the airframe and coupling the pair of skids to each other, the cross tube including an internal space; and a stiffening portion configured to suppress flattening of the cross tube 30), wherein: the cross tube includes curved pipe parts located closer to corresponding end portions of the cross tube than corresponding attached portions of the cross tube, the attached portions being configured to be attached to the airframe; characterized in that the stiffening portion is arranged in at least one of internal spaces of pipe parts (37, 32) that are the curved pipe parts and an internal space of a pipe part located between the pair of curved pipe parts and said stiffening portion includes an enlarged diameter portion (41, 42, 43) and a fastening portion (60, 61, 64), wherein said enlarged diameter portion (41, 42, 43) is configured to increase in diameter by axial fastening power of the fastening portion (60, 61, 64) and to be brought into contact with an inner surface of the cross tube, and said fastening portion (60, 61, 64) is configured to generate said axial fastening power with respect to said enlarged diameter portion (41, 42, 43), the cross tube includes curved pipe parts located closer to corresponding end portions of the cross tube than corresponding attached portions of the cross tube, the attached portions being configured to be attached to the airframe; characterized in that the stiffening portion is arranged in at least one of internal spaces of pipe parts (37, 32) that are the curved pipe parts and an internal space of a pipe part located between the pair of curved pipe parts and said stiffening portion includes an enlarged diameter portion (41, 42, 43) and a fastening portion (60, 61, 64), wherein said enlarged diameter portion (41, 42, 43) is configured to increase in diameter by axial fastening power of the fastening portion (60, 61, 64) and to be brought into contact with an inner surface of the cross tube, and said fastening portion (60, 61, 64) is configured to generate said axial fastening power with respect to said enlarged diameter portion (41, 42, 43), a ring capable of increasing in diameter and configured to increase in diameter to be brought into contact with an inner surface of the pipe part (37, 32), a first disc and second disc internally fitted to the ring and configured to sandwich the ring in an axial direction, and a wire configured to generate axial fastening power with respect to the first disc and the second disc; the first disc includes a first engaging portion which is brought into contact with the ring to restrict the displacement of the ring toward one side in the axial direction and increase the diameter of the ring; and the second disc includes a second engaging portion which is brought into contact with the ring to restrict the displacement of the ring toward the other side in the axial direction and increase the diameter of the ring.