Plug for expanding end of metal pipe, method for expanding pipe using plug, and method for manufacturing metal pipe

Provided is a plug (30) which is capable of reducing irregularities in the inner diameter at a frontmost end portion of an end of a metal pipe in the vicinity of an opening. The plug is provided with a taper portion (301) and a cylindrical portion (302) continuous therewith. The diameter of a tapered surface (301S) gradually increases from the front end of the plug toward the rear end. The tapered surface (301S) includes a contact surface portion (CS), a terminal point surface portion (ES), and a start point surface portion (OS). The contact surface portion (CS) has a diameter identical to the inside diameter of the metal pipe before the metal pipe is expanded. The contact surface portion further has a taper angle of 1.0 to 3.0°. The terminal point surface portion (ES) is located at the rear end of the tapered surface (301S) and has a diameter of D1. The start point surface portion (OS) is located between the contact surface portion (CS) and the terminal point surface portion (ES) and has a diameter of D2 = D1 x 0.99. The axial distance LR between the start point surface portion (OS) and the terminal point surface portion (ES) satisfies the expression (1). 22≤LR/((D1 – D2)/2)≤115 (1)