A flow rate control device includes: a fuel gas supply channel having a flow rate adjusting valve; a thermal type mass flow rate sensor arranged in the supply channel; a calculation unit which calculates a thermal quantity flow rate of the fuel gas defined by a product of a volume flow rate of the fuel gas and a heat generation quantity per unit volume of the fuel gas according to an output from the thermal type sensor; and a flow rate control unit which controls the open degree of the flow rate adjusting valve according to a difference between a control target value for controlling the thermal quantity flow rate and the thermal quantity flow rate calculated by the calculation unit.