A control method for a fuel cell system, in which fuel gas and oxidant gas are supplied to a fuel cell to generate electric power, a load is driven by supplying the generated power to a load circuit, and the generated power from the fuel cell is stored in a power storage unit, comprising the steps of: (i) setting and storing in advance a value of margin load power, which is an increment of the electric power in addition to a required load power required to drive the load, to be supplied from the power storage unit and the fuel cell to the load when the load of the load circuit is increased at a predetermined rate; (ii) computing electric power capable of being inputted and stored in the power storage unit, and computing outputable power that said power storage unit can output at present based on a temperature and charging level thereof; (iii) comparing the margin load power and the outputable power, (iv)(a) setting a target power generation amount, which is a target of electric power generated from the fuel cell, to be equal to required load power in case the margin load power is smaller than outputable power, (iv)(b) computing an electric power difference between the margin load power and the outputable power in case the margin load power is not smaller than the outputable power, after having computed the electric power difference between the margin load power and the outputable power, comparing said inputable power with the electric power difference between the margin load power and the outputable power, (iv) setting a target charge amount to be equal to the electric power difference in case the electric power difference is smaller than the inputable power, setting the target power generation amount to be equal to a sum of the required load power and the target charge amount, (iv) setting the target charge amount to be equal to the inputable power in case the electric power difference is not smaller than inputable power, setting the target power generation amount to be equal to the sum of the required load power and the target charge amount; and (v) computing a target flow rate and target pressure of both the fuel gas and the oxidant so as to obtain said target power generation amounts. (i) setting and storing in advance a value of margin load power, which is an increment of the electric power in addition to a required load power required to drive the load, to be supplied from the power storage unit and the fuel cell to the load when the load of the load circuit is increased at a predetermined rate; (ii) computing electric power capable of being inputted and stored in the power storage unit, and computing outputable power that said power storage unit can output at present based on a temperature and charging level thereof; (iii) comparing the margin load power and the outputable power, (iv)(a) setting a target power generation amount, which is a target of electric power generated from the fuel cell, to be equal to required load power in case the margin load power is smaller than outputable power, (iv)(b) computing an electric power difference between the margin load power and the outputable power in case the margin load power is not smaller than the outputable power, after having computed the electric power difference between the margin load power and the outputable power, comparing said inputable power with the electric power difference between the margin load power and the outputable power, (iv) setting a target charge amount to be equal to the electric power difference in case the electric power difference is smaller than the inputable power, setting the target power generation amount to be equal to a sum of the required load power and the target charge amount, (iv) setting the target charge amount to be equal to the inputable power in case the electric power difference is not smaller than inputable power, setting the target power generation amount to be equal to the sum of the required load power and the target charge amount; and (v) computing a target flow rate and target pressure of both the fuel gas and the oxidant so as to obtain said target power generation amounts.