Current-voltage (I-V) characteristics and electrode impedance of a tubular-type solid oxide fuel cell (SOFC) were analyzed for mixed fuel gases, consisting mainly of CO, H-2, H2O, and a carrier gas, as simulated reformed gas of hydrocarbons or coal gas. I-V characteristics of a single cell were measured as a function of various operational parameters including the H-2-to-CO ratio, the type of carrier gas such as He, N-2, and Ar, the temperature, the fuel-to-carrier gas ratio, and the water vapor concentration. It has been experimentally confirmed that the use of CO-rich gases results in comparable performance to that of H-2-rich gases and thus mixed gas such as coal gas is useful as a SOFC fuel. We have found, for the first time, that the I-V characteristics depend on the carrier gas, indicating the importance of gas transport in porous anodes for anodic polarization. The change in cell voltage by varying fuel compositions was mainly caused by the change in anode impedance associated with a low frequency semicircle in a Cole-Cole impedance plot at 1000degreesC. The fuel gas compositions in thermodynamic equilibrium were calculated and compared with the initial gas compositions, suggesting the importance of water vapor concentration to control the equilibrium H-2-to-CO ratio for CO-rich fuel gases.