A method for manufacturing an all-solid-state battery includes a battery unit producing step, a flattening step, and a stacking step. In the battery unit producing step, a battery unit having a plate shape is produced through a pressing step in which a laminate including at least one each of a positive electrode active material layer, a solid electrolyte layer, and a negative electrode active material layer is pressed in a thickness direction of the laminate with a first pressure. In the flattening step, the battery unit is flattened by pressing the produced battery unit in the thickness direction with a second pressure equal to or lower than the first pressure while heating the battery unit to a temperature equal to or higher than a temperature at which the battery unit softens and is deformed. In the stacking step, a plurality of the flattened battery units are stacked.