To provide a wiring formation method that can increase the wiring density in a case where wiring is formed on an inclined surface by three-dimensional additive manufacturing. The wiring formation method of the present disclosure includes a metal member forming step of forming multiple metal members with a first fluid containing metal particles, a resin layer forming step of forming a resin layer including an upper surface and an inclined surface inclined downward from the upper surface, and a connection wiring forming step of forming multiple connection wirings on the inclined surface and the upper surface of the resin layer with a second fluid containing metal particles, and the connection wirings being formed to individually connect the multiple connection wirings to the multiple metal members on a lower surface of the inclined surface.