1. A method for firing a copper paste, the method comprising:an application step of applying a copper paste to a substrate, the copper paste comprising copper particles, a binder resin, and a solvent;
after the application step, a first heating step of heating the substrate at 350° C. or more and 500° C. or less in a nitrogen gas atmosphere containing 500 ppm or more and 2000 ppm or less of oxidizing gas in a volume ratio, and oxidizing and sintering the copper particles in the copper paste; and
after the first heating step, a second heating step of heating the substrate at 400° C. or more and 550° C. or less in a nitrogen gas atmosphere containing 1% or more reducing gas in a volume ratio, and reducing the oxidized and sintered copper oxide,
wherein a mass percent of the binder resin in an organic vehicle contained in the copper paste is more than 0.05% and less than 17.0%,
wherein the binder resin is one or more selected from a group consisting of cellulose resin, acrylic resin, butyral resin, alkyd resin, epoxy resin and phenol resin,
wherein the copper particles in the copper paste are oxidized by oxidation in the first heating step, and cuprous oxide is formed in a continuous shape so as to cover the copper particle surface, and
wherein the oxide of copper particles formed by the first heating step includes cuprous oxide and cupric oxide, and heating is carried out such that an amount of the cuprous oxide is larger than an amount of the cupric oxide,
wherein an electrical resistivity of a copper wiring obtained after the second heating step is 6 ??cm or less.