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 (S) 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 (S) 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 (S) 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. <EOS>