A fuel cell receiving hydrogen and oxygen to generate electric power; A hydrogen supply line for recirculating hydrogen discharged from the fuel cell and supplying it to the fuel cell; A time measuring unit that measures a duration from a point in time when the operation of the fuel cell is terminated to a time when the fuel cell is restarted; An air volume estimating unit for estimating the amount of air introduced into the fuel cell during the duration using the duration measured by the time measuring unit; And a concentration estimating unit that estimates the hydrogen concentration of the hydrogen supply line when the fuel cell is restarted based on the duration measured by the time measurement unit and the amount of air introduced by the air quantity estimating unit. And estimation methods are introduced.
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