NOVELTY - A closed type battery comprises a battery element covered with a covering film (4) and a heat fusion seal portion formed by heat fusion on a periphery of the covering film, where a cleaving strength upon an internal pressure rise of a seal portion positioned between a positive electrode terminal and a negative electrode terminal is larger than that of any other seal portion, and a safety valve adapted to release pressure upon a battery's internal pressure rise is located at a portion other than the inter-terminal seal portion.    USE - As closed type battery (claimed) such as a lithium ion battery used in cellular phones, notebook PCs, video cameras, and in electric cars or vehicles.    ADVANTAGE - According to the closed type battery, the strength of a cleavage due to an increase in its internal pressure of a heat fusion seal portion that forms a seal portion positioned between a positive and a negative electrode terminal is increased, and a safety valve is provided at a seal portion other than that between the positive and the negative electrode terminal which also provides a site ensuring a smooth operation even when several batteries are stacked together, thereby ensure to expel internal gases from them by way of the safety valve. The assembled battery comprising several lithium ion batteries or other closed type batteries connected together. When that assembled battery is left overcharged for some unknown reasons to cause gases to be generated in it, resulting in an increase in its internal pressure or causing battery elements to generate heat, it is possible to release pressure by way of the safety valve, thereby preventing the battery from bursting up or finding where gases are being released, so that the generated gases can be collected. It is thus possible to obtain an assembled battery with improved safety comprising lithium ion batteries.    DESCRIPTION OF DRAWING(S) - The figure shows a plan view of a closed type battery.    Lithium ion battery (1)    Positive electrode terminal (2)    Negative electrode terminal (3)    Covering film (4)    Film (41)    Flat sheet-form film (42)