Abstract: In a method for heat treatment of an existing pipe constituting a piping system, for converting the residual stress at a welded metallic portion and a welding heat influenced portion of the inside surface of the pipe, into a compressive stress and thereby generate the compressive stress in the inside surface of the existing pipe, a coolant is retained in the pipe; an arbitrary portion of the outside surface of the pipe is heated; thereby a temperature distribution little in temperature difference is produced in the wall surface of the pipe at the heated portion; and then the coolant is allowed to flow. By converting the residual stress at the welded metallic portion and the welding heat influenced portion of the inside surface of the pipe, into the compressive stress, stress corrosion cracking generated from the welded metallic portion and the welding heat influenced portion can be suppressed.