The present invention relates to a core substrate, a manufacturing method thereof, and a structure for a metal via. In accordance with an embodiment of the present invention, a core substrate including: an insulation layer; a plurality of metal vias passing through the insulation layer and formed to become wider from upper and lower surfaces to a middle part of the insulation layer; and a conductive layer formed on the upper and lower surfaces of the insulation layer and connected to the plurality of metal vias. Further, a manufacturing method thereof and a structure for a metal via are provided.