A circuit line forming device, including an inkjet head to eject a conductive ink onto one side of a substrate, the conductive ink containing nanoparticles including a ferromagnetic core and a conductive layer surrounding the ferromagnetic core; and a magnetic field generator part, positioned on the other side of the substrate in correspondence with the inkjet head, wherein the magnetic field generator part applies a magnetic field on the conductive ink, when the conductive ink is ejected to form circuit lines.