INSULATOR

An insulator capable of improving manufacturability and rigidity is provided. An insulator is provided in a fuel cell stack. The fuel cell stack includes: a cell stack body formed by stacking a plurality of power generation cells, each including an electrolyte membrane-electrode assembly and separators and; a terminal plate provided at an end of the cell stack body in the stacking direction; the insulator; and an end plate. The terminal plate, the insulator, and the end plate are provided in this order from the cell stack body side in the stacking direction. The insulator includes: a main body; a first rib standing from a first surface of the main body, the first surface facing the terminal plate; and a second rib standing from a second surface of the main body, the second surface facing the end plate.