NOVELTY - A fuel-cell apparatus consists of a fuel cell (1), a modification portion (4), a raw-material-and-mineral-fuel supplier (2), an oxygen-containing-gas supplier (3), a water supplier (11), and a water-treatment unit. During steady operation, a controller (14) controls so that the raw materials, mineral fuel, and water undergo steam reforming. When the controller determines deterioration of water quality based on a detector (21), the controller controls so that raw material, mineral fuel, and oxygen-containing gas undergo partial oxidation reforming.    USE - Fuel-cell apparatus is used for electric-power generation.    ADVANTAGE - The fuel-cell apparatus maintains high modification performance.    DETAILED DESCRIPTION - A fuel-cell apparatus consists of a fuel cell (1), a modification portion (4) for producing fuel gas for fuel cell, a supplier (2) for supplying raw material and mineral fuel to the modification portion, a supplier (3) for supplying oxygen-containing gas to the modification portion, a supplier (11) for supplying water to the modification portion, and a water-treatment unit for processing water supplied to the modification portion. During steady operation, a controller (14) controls so that the raw materials, mineral fuel, and water supplied to the modification portion undergo steam reforming. A water-quality detector (21) is provided between the water-treatment unit and modification portion. When the controller determines deterioration of water quality based on the detector, the controller controls so that raw material, mineral fuel, and oxygen-containing gas supplied to the modification portion undergo partial oxidation reforming.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the fuel-cell apparatus. (Drawing includes non-English language text)    Fuel cell (1)    Raw-material-and-mineral-fuel supplier (2)    Oxygen-containing-gas supplier (3)    Modification portion (4)    Water supplier (11)    Controller (14)    Water-quality detector (21)