A fuel cell system comprising: a reformer that reforms raw fuel to generate fuel gas; a solid oxide fuel cell that generates electric power by using said fuel gas and oxidant gas; a housing that houses said reformer and said fuel cell in an internal space thereof; a raw fuel supply part that supplies said raw fuel to said reformer; an oxidant gas supply part that supplies said oxidant gas to said fuel cell; an exhaust gas combustor that burns unutilized fuel gas discharged in an unutilized state from said fuel cell; a startup controller that controls said raw fuel supply part to increase an amount of said raw fuel supplied from said raw fuel supply part in stages during a startup operation performed until an output of said fuel cell reaches a steady operation output; and a heater that heats an interior of said housing during said startup operation, characterized in that, for the amount of said raw fuel supplied from said raw fuel supply part during said startup operation, a plurality of set supply amounts are preset in stages within a range less than or equal to a steady supply amount corresponding to said steady operation output, said startup controller controls said raw fuel supply part to repeat a process of maintaining the amount of said raw fuel supplied from said raw fuel supply part at one set supply amount among said plurality of set supply amounts in sequence, starting from the smallest set supply amount, and after the output of said fuel cell has reached a set output corresponding to said one set supply amount, increasing the amount of said raw fuel to a set supply amount that is the next smallest after said one set supply amount, an initial supply amount that is the smallest set supply amount among said plurality of set supply amounts is greater than or equal to a first supply amount that is a supply amount of said raw fuel corresponding to auxiliary driving power necessary to drive auxiliary machinery that includes said raw fuel supply part, said oxidant gas supply part, and said heater, during said startup operation, electric power is supplied from an electric power system of an installation where said fuel cell system is installed to said auxiliary machinery, and in a case where a power failure has occurred in said electric power system during said startup operation, electric power is supplied from said fuel cell to said auxiliary machinery under control of said startup controller. a reformer that reforms raw fuel to generate fuel gas; a solid oxide fuel cell that generates electric power by using said fuel gas and oxidant gas; a housing that houses said reformer and said fuel cell in an internal space thereof; a raw fuel supply part that supplies said raw fuel to said reformer; an oxidant gas supply part that supplies said oxidant gas to said fuel cell; an exhaust gas combustor that burns unutilized fuel gas discharged in an unutilized state from said fuel cell; a startup controller that controls said raw fuel supply part to increase an amount of said raw fuel supplied from said raw fuel supply part in stages during a startup operation performed until an output of said fuel cell reaches a steady operation output; and a heater that heats an interior of said housing during said startup operation, characterized in that, for the amount of said raw fuel supplied from said raw fuel supply part during said startup operation, a plurality of set supply amounts are preset in stages within a range less than or equal to a steady supply amount corresponding to said steady operation output, said startup controller controls said raw fuel supply part to repeat a process of maintaining the amount of said raw fuel supplied from said raw fuel supply part at one set supply amount among said plurality of set supply amounts in sequence, starting from the smallest set supply amount, and after the output of said fuel cell has reached a set output corresponding to said one set supply amount, increasing the amount of said raw fuel to a set supply amount that is the next smallest after said one set supply amount, an initial supply amount that is the smallest set supply amount among said plurality of set supply amounts is greater than or equal to a first supply amount that is a supply amount of said raw fuel corresponding to auxiliary driving power necessary to drive auxiliary machinery that includes said raw fuel supply part, said oxidant gas supply part, and said heater, during said startup operation, electric power is supplied from an electric power system of an installation where said fuel cell system is installed to said auxiliary machinery, and in a case where a power failure has occurred in said electric power system during said startup operation, electric power is supplied from said fuel cell to said auxiliary machinery under control of said startup controller.