NOVELTY - The device (5) has a first gas diffusion resistance calculation section for calculating first gas diffusion resistance, which shows a difficulty of diffusing reaction gas in an inner side of a fuel cell (1). A second gas diffusion resistance calculation section calculates second diffusion resistance on the basis of proton transfer resistance, while showing diffusion characteristics varying due to a dried-up state in the inner side of the fuel cell, where the first calculation section calculates the first resistance by subtracting the second diffusion resistance from the first resistance.    USE - Device for monitoring operational conditions of a fuel cell for generating electric power for an electric load e.g. electric motor and rechargeable battery, of an electric vehicle i.e. fuel cell vehicle.    ADVANTAGE - The device includes a water content calculation section, which calculates the water content in the inner side of the fuel cell on the basis of the first diffusion resistance, where a recovery control section performs water content recovery process such that the water content in the inner side of the fuel cell is within an optimum water content range when the water content of the inner side of the fuel cell calculated by the water content calculation section is out of the optimum water content range, thus detecting the water content in the inner side of the fuel cell with high accuracy, correctly detecting excess state of the water content in the inner side of the fuel cell, and suppressing fluctuation of the output of the fuel cell and hence driving the fuel cell with high efficiency.    DETAILED DESCRIPTION - The reaction gas is selected from a group consisting of a fuel gas such as hydrogen gas and oxidizing gas such as oxygen gas.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a fuel cell system including a fuel cell and a fuel cell monitoring device.    Fuel cell (1)    Fuel cell monitoring device (5)    High pressure hydrogen tank (32)    Water pump (41)    Radiator (42)    Temperature sensor (43)