Plug for tube expansion the end of the metal tube production method of the expanded methods and metal tube using a plug

Providing a plug (30) which can reduce variation in inner diameter of the leading edge portion of the vicinity of the opening of the end of the metal tube. Plug is continuously provided with a tapered portion (301) cylindrical part and a (302). The diameter of the tapered surface (301S) gradually increases toward the rear end from the front end of the plug. Tapered surface (301S) comprises contacting surface portion and (CS), the end point surface portion and (ES), starting surface portion and (OS). Contact surface section (CS) has a diameter as large as the inner diameter of the metal pipe before pipe expansion. Contact surface section (CS) further comprises a taper angle of 1.0 to 3.0 °. Endpoint surface portion (ES) is located at the rear end of the tapered surface (301S), it has a diameter D1. Starting surface portion (OS) is located between the contact surface part and (CS) ending surface portion and (ES), it has a diameter D2 = D1 × 0.99. The axial distance LR between the start surface portion and (OS) endpoint surface portion and (ES) satisfies the formula (1). 22 ≦ LR / ((D1-D2) / 2) ≦ 115 (1)