A plurality of beads protruding from an inner face of the tube are provided in such a manner that the beads are arranged at a predetermined pitch in an axial direction of the tube; and a circumference of the tube is divided at least into thirds, and the beads are aligned in a circumferential direction of the tube; and the beads aligned in the circumferential direction of the tube are provided at plural rows at the predetermined pitch in the axial direction of the tube, and the beads adjoining in the axial direction are shifted by substantially a half of a circumferential length of the bead to one another. Alternatively, the circumference of the tube is divided into parts of an even number of four or more, and the beads are aligned in the circumferential direction so as to be alternately formed in the parts of the circumference.