The core metal for resin welding improves bonding strength between an outer peripheral resin member and the core metal. The outer peripheral resin member 30 is welded to the peripheral surface 21 of the core metal 20 by fitting the core metal into a fitting hole 31 of the outer peripheral resin member 30, preparing the outer peripheral resin member 30 providing the fitting hole 31 with a smooth internal wall surface; preparing the core metal 20 wherein streaky protrusions 26 and smooth portions 25 are provided; allowing the top parts of the protrusions 26 to contact, and the smooth portions of the core metal 20 to face the smooth internal wall surface; and induction heating to weld the outer peripheral resin member 30 to the protrusions 26 and the smooth portions 25.