PURPOSE: To provide a fuel cell capable of keeping the insulation between a cell stack and a gas manifold and stably generating electricity, and an external diagnostic method therefor.

CONSTITUTION: A gas manifold for supplying and discharging fuel and oxidized gas is provided on the side surface of a cell stack formed by plurally laminating unit cells and separators. A drain pot 12 for accumulating the drain generated in the manifold is mounted on the bottom plate lower part of the gas manifold, and the drain pot is set to have the same potential as the manifold.