PURPOSE: To reduce heat-treatment irregularity in the vicinity of an air pocket, by heating a metal pipe in a water filled-up state to generate nucleate boiling or film boiling in the air pocket and expelling residual air of the air pocket by boiling steam.

CONSTITUTION: The vicinity of the expanded hollow part 8 of a metal pipe 1 brought to a state filled up with water receives high frequency induction heating by a coil X and the temp. of the pipe wall in the vicinity of a welded part 10 is raised as shown by time ToWTn. Because the cooling water of a ring shaped hollow part 7 is held to a stagnant state, the temp. at a model point A becomes higher than that at a model point B but, when the model point B reaches temp. generating nucleate boiling on the inner surface of a double pipe part 3 at time T1 by further raising temp., almost constant temp. succeeds because heat of evaporation of water is large during a time when residual air is present in an air pocket. The residual air of the air pocket 9 is gradually expelled along with generated boiling steam.