An electric resistance welded pipe welding apparatus (50; 70) for manufacturing an electric resistance welded pipe, in combination with an open pipe, the electric resistance welded pipe welding apparatus (50; 70) being configured to weld a join portion of two end parts (2a, 2b) of an open pipe that includes the opening extending in a running direction, the two end parts (2a, 2b) facing the opening, the electric resistance welded pipe welding apparatus (50; 70) further comprising: an induction coil formed so as to encircle an outer circumferential face of the open pipe and configured such that the two end parts (2a, 2b) are melted by an induction current generated by a high frequency current passing through the induction coil, the electric resistance welded pipe welding apparatus (50; 70) being configured such that the two end parts (2a, 2b) are placed in contact with each other and welded together at the join portion while gradually closing a gap of the opening, wherein: the electric resistance welded pipe welding apparatus (50; 70) further comprises a ferromagnet (8; 9) including an inside section (8a; 9a) arranged to extend in the running direction inside the open pipe and inside the induction coil, an outside section (8b; 9b) arranged to extend in the running direction outside the open pipe and outside the induction coil, and a center section (8c; 9c) extending in a pipe inside-outside direction of the open pipe between the inside section (8a; 9a) and the outside section (8b; 9b) at a position at an upstream side of the induction coil; and in a state in which an open side of a portion of the ferromagnet (8; 9) configured by portions of the inside section (8a; 9a) and the outside section (8b; 9b) further toward the downstream side than the center section (8c; 9c), and by the center section (8c; 9c), is disposed facing the running direction downstream side, a closed circuit of magnetic flux is formed passing through the inside section (8a; 9a), the center section (8c; 9c), and the outside section (8b; 9b). an induction coil formed so as to encircle an outer circumferential face of the open pipe and configured such that the two end parts (2a, 2b) are melted by an induction current generated by a high frequency current passing through the induction coil, the electric resistance welded pipe welding apparatus (50; 70) being configured such that the two end parts (2a, 2b) are placed in contact with each other and welded together at the join portion while gradually closing a gap of the opening, wherein: the electric resistance welded pipe welding apparatus (50; 70) further comprises a ferromagnet (8; 9) including an inside section (8a; 9a) arranged to extend in the running direction inside the open pipe and inside the induction coil, an outside section (8b; 9b) arranged to extend in the running direction outside the open pipe and outside the induction coil, and a center section (8c; 9c) extending in a pipe inside-outside direction of the open pipe between the inside section (8a; 9a) and the outside section (8b; 9b) at a position at an upstream side of the induction coil; and in a state in which an open side of a portion of the ferromagnet (8; 9) configured by portions of the inside section (8a; 9a) and the outside section (8b; 9b) further toward the downstream side than the center section (8c; 9c), and by the center section (8c; 9c), is disposed facing the running direction downstream side, a closed circuit of magnetic flux is formed passing through the inside section (8a; 9a), the center section (8c; 9c), and the outside section (8b; 9b). the electric resistance welded pipe welding apparatus (50; 70) further comprises a ferromagnet (8; 9) including an inside section (8a; 9a) arranged to extend in the running direction inside the open pipe and inside the induction coil, an outside section (8b; 9b) arranged to extend in the running direction outside the open pipe and outside the induction coil, and a center section (8c; 9c) extending in a pipe inside-outside direction of the open pipe between the inside section (8a; 9a) and the outside section (8b; 9b) at a position at an upstream side of the induction coil; and in a state in which an open side of a portion of the ferromagnet (8; 9) configured by portions of the inside section (8a; 9a) and the outside section (8b; 9b) further toward the downstream side than the center section (8c; 9c), and by the center section (8c; 9c), is disposed facing the running direction downstream side, a closed circuit of magnetic flux is formed passing through the inside section (8a; 9a), the center section (8c; 9c), and the outside section (8b; 9b).