A battery pack comprising: a laminated body in which unit cells are laminated one on another; a cell case made of a rigid material and having a first opening and containing the laminated body; and a first lid member made of a flexible material and tightly closing the first opening, wherein: the first opening is positioned to face a first face of the laminated body in connection with a laminated direction of the unit cells; and the first lid member is configured to: be deformable while tightly closing the first opening; be positioned away from the first face of the laminated body if an internal pressure of the cell case is equal to an external pressure of the cell case; and if the internal pressure of the cell case is lower than the external pressure of the cell case, deform, come into contact with the first face of the laminated body, and apply a pressure based on a differential pressure between the internal pressure of the cell case and the external pressure of the cell case to the contacting face. a laminated body in which unit cells are laminated one on another; a cell case made of a rigid material and having a first opening and containing the laminated body; and a first lid member made of a flexible material and tightly closing the first opening, wherein: the first opening is positioned to face a first face of the laminated body in connection with a laminated direction of the unit cells; and the first lid member is configured to: be deformable while tightly closing the first opening; be positioned away from the first face of the laminated body if an internal pressure of the cell case is equal to an external pressure of the cell case; and if the internal pressure of the cell case is lower than the external pressure of the cell case, deform, come into contact with the first face of the laminated body, and apply a pressure based on a differential pressure between the internal pressure of the cell case and the external pressure of the cell case to the contacting face. the first opening is positioned to face a first face of the laminated body in connection with a laminated direction of the unit cells; and the first lid member is configured to: be deformable while tightly closing the first opening; be positioned away from the first face of the laminated body if an internal pressure of the cell case is equal to an external pressure of the cell case; and if the internal pressure of the cell case is lower than the external pressure of the cell case, deform, come into contact with the first face of the laminated body, and apply a pressure based on a differential pressure between the internal pressure of the cell case and the external pressure of the cell case to the contacting face. be deformable while tightly closing the first opening; be positioned away from the first face of the laminated body if an internal pressure of the cell case is equal to an external pressure of the cell case; and if the internal pressure of the cell case is lower than the external pressure of the cell case, deform, come into contact with the first face of the laminated body, and apply a pressure based on a differential pressure between the internal pressure of the cell case and the external pressure of the cell case to the contacting face.