The present invention provides a method for forming a through-via, including the steps of (1) forming an alloy film as a diffusion-preventive layer that prevents diffusion of copper, in an area on a side wall of a hole formed in a substrate that extends from an entrance of the hole to a central part of the hole, by use of an electroless cobalt plating solution or an electroless nickel plating solution containing at least cobalt ion or nickel ion, a complexing agent, a reductant, and a pH adjusting agent; (2) forming an alloy film as a diffusion-preventive layer in an area on the side wall of the hole formed in the substrate that extends from the central part of the hole to a bottom of the hole, by use of an electroless cobalt plating solution or an electroless nickel plating solution containing at least the cobalt ion or the nickel ion, the complexing agent, the reductant, the pH adjusting agent, and an amino group-containing polymer; and (3) stacking a copper seed layer on the diffusion-preventive layer formed in each of steps (1) and (2) by use of an electroless copper plating solution.