A co-fired all-solid-state battery that includes a negative electrode, a solid electrolyte layer, and a positive electrode. The negative electrode contains a negative electrode active material and a garnet-type solid electrolyte. The negative electrode active material contains Li, V, and O. The negative electrode active material has a mole ratio (Li/V) of a Li content to a V content of 2.0 or more. The garnet-type solid electrolyte contains Li, La, Zr, and O.