A steel pipe according to the present invention has a waveform shape formed on an outer diameter thereof by a pipe expansion process. The value "a/w" is 0.038% or smaller, where "a" and "w" denote the amplitude and the wavelength of the waveform shape, respectively. It is desirable to arrange the value "w/»" indicating a ratio of the wavelength "w" of the waveform shape to a Timoshenko's buckling wavelength "»" so as to be larger than 0.8. As a result, it is possible to provide a steel pipe of which deformability is improvable, while reducing labor and time required by the pipe expansion process.