To provide a means for manufacturing a spiral steel pipe having a sound weld zone by securing a proper welding gap when the spiral steel pipe is manufactured by using a steel strip, whose width is not necessarily uniform, as a base material.

In the method of manufacturing the spiral steel pipe, a steel strip is trimmed on the side, formed into a spiral shape and abutted on the edges, with the inner face and then the outer face welded. The variance of the width of the steel strip is continuously measured on the upstream side at least by 2D portion (D is the outer diameter of the spiral pipe to be manufactured) from an inner face welding point. At the same time, guidance is provided so that a prescribed welding gap is obtained at the inner welding point in accordance with the variance of the steel strip width.