An optical fiber temperature sensor implements a temperature monitoring function in an interphase insulating material between sandwich bus bars. The optical fiber temperature sensor is formed by housing an optical fiber cable in a housing formed from an ultra-thin sheet made from an insulating material having insulation quality equal to or higher than interphase insulating material between sandwich bus bars. Multiple ultra-thin columnar members, each made from same material as is housing, are housed in multiple locations in housing. Multiple sensor rings are each formed by unfixedly winding a portion of optical fiber cable with a length equal to or longer than that corresponding to range resolution around corresponding one of ultra-thin columnar members. The multiple sensor rings measure temperatures in multiple locations. Silicone sealing is applied to housing side surfaces. All surfaces of housing are sealed with a hermetic sealing member made from a silicone-based liquid insulating material.