Abstract: An energy storage device includes an electrode unit in which a cathode having a cathode lead, an anode having an anode lead, and a separator located between the cathode and the anode to separate the cathode and the anode from each other are rolled together; a housing receiving the electrode unit; an electrolyte filled in the housing; an inner terminal arranged in the housing to face the electrode unit; and an outer terminal connected to the inner terminal. A groove is formed in a side of the inner terminal, and a side protrusion is formed on an inner wall of the housing at a location corresponding to the groove.