FUEL CELL SYSTEM, AND METHOD OF ESTIMATING HUMIDITY IN EXHAUST GAS THEREFROM

A fuel cell system includes a mass flow rate measuring unit, an oxygen consumption mass flow rate acquiring unit, an exhaust-side air temperature acquiring unit, and an exhaust-side air humidity estimating unit. The mass flow rate measuring unit measures a first mass flow rate of intake-side air and a second mass flow rate of exhaust-side air. The oxygen consumption mass flow rate acquiring unit acquires a mass flow rate of oxygen consumption. The exhaust-side air humidity estimating unit estimate humidity in the exhaust-side air, on the basis of a difference between a flow rate of intake gas in the fuel cell system and a flow rate of exhaust gas from the fuel cell system, and the mass flow rate of the oxygen consumption.