This metal tube (1) has a circular cross section having an outer diameter D, and the length thereof is 6D or more. The metal tube (1) comprises a high-strength portion (1A) and a low-strength portion (1B). The high-strength portion (1A) is disposed around the circumstance of the metal tube (1) in a range of (2/3)D-3D of the longitudinal dimension of the metal tube (1). The yield strength of the high-strength portion (1A) is 500 Mpa (tensile strength: 980 Mpa) or more. The low-strength portion (1B) is disposed around the circumstance of the metal tube (1) on both sides of the high-strength portion (1A) in the metal-tube longitudinal direction. The yield strength of the low-strength portion (1B) is 60% to 85% of the high-strength portion (1A).