A method for manufacturing a battery assembly provided by the present invention includes a step of measuring a stacking direction length of a stacked body including a predetermined number of unit cells (12) constituting a battery assembly (10) and arranged in the stacking direction; and a step of bundling a body (20) to be bundled that includes the stacked body. Here, the body to be bundled is provided with length adjusting means (40) for converging a spread in stacking direction length. The bundling step is implemented by setting the length adjusting means according to the stacking direction length of the stacked body, so that a length of the battery assembly in the stacking direction is a stipulated length (LT) and a bundling pressure of the body to be bundled is a stipulated pressure.