Disclosed is a separator for a fuel cell stack. The separator includes a plate having a first opening and a second opening on a surface thereof. A field or a channel is formed on the surface of the plate. The field has a first end and a second end. A field inlet is formed on the surface of the plate and connects the first end of the field to the first opening, and a field outlet is formed on the surface of the plate and connects the second end of the field to the second opening. An area of a cross-section of the field inlet, which is cut perpendicular to the surface of the plate, is smaller than an area of a cross-section of the field outlet, which is cut perpendicular to the surface of the plate.