To set a current inoperative more than a specific value at room temperature by making a current flowing conductive member such as a fixing plate and a movable plate, using material whose conductivity is more than a specific value in standard conductivity of international anneal copper(IACS).

A material of a movable plate 3d of a thermal protector used in a lithium-ion secondary battery pack is set to have more than 50% IACS (for example, beryllium copper 11). The materials of a fixing plate 3a containing a first external terminal 3b, and a second external terminal 3f are copper having more tan 95% IACS. Thus, heat generation in a thermal protector is little, and discharge efficiency of the generated heat increases. The resistance between the external terminals becomes about 2.5 mQ, inoperative current becomes more than 12A at room temperature and more than 7A at 60°C, and therefore, a battery pack having larger battery capacity can be installed in a personal computer.