PURPOSE: To provide the layer built fuel cell, in which the electrolyte supply time can be judged accurately.

CONSTITUTION: In a layer built fuel cell 1, which consists of plural laminated unit-cells 2A-2G, unit-cells 2B, 2F are selected at random as monitor cells, and these monitor cells 2A, 2B are structured so that the lowering of the performance thereof due to the splash of phosphoric acid (electrolyte) is generated earlier than other unit-cells 2A, 2C, 2D, 2E, 2G. Voltmeters (detecting means) V1, V2 for detecting the output voltage of the monitor cells 2B, 2F are provided, and while the monitor cells 2B 2F are structured so as to be electrically separated from other unit-cells 2A, 2C, 2D, 2E, 2G freely. At the time when the output voltage of the monitor cells 2B, 2F is lowered to a predetermined value or less, phosphoric acid is supplied, then, since the over-splash of phosphoric acid is not generated in other unit-cells 2A, 2C, 2D, 2E, 2G at this stage, the output voltage of the fuel cell 1 is recovered in a short time.