A micro-coil can be fabricated by turning a thin wire on core wire, but the plastic deformation is difficult to be induced into such thin wire due its higher strength compared with bulk. Then, it is necessary to soften a micro-wire for improving the ability of the plastic deformation. In this paper, we heat-treated the copper micro-wire using Joule heat, and it was soften in very short time. Using the heat-treated copper micro-wire, we made a micro-coil. The ratio of the diameter of the coil to the diameter of the wire used was only 3, and the electromagnetic function of the fabricated micro-coil was investigated.