A clad material of a clad steel pipe is handled on at least one of an internal surface and external surface of a base material, this clad material has two lateral edges, these two edges are welded together in order to form the pipe. In a lengthwise direction, the thickness of the clad material at the two ends of the clad material is predetermined to be at a first value. This first value enables absorption of a discrepancy in thickness when connecting the clad steel pipe to another one; and a thickness of the clad material at other parts other than the two ends is determined to be at a second value. This second value is smaller than the first one.