A fuel cell system connected with a load, comprising: a fuel cell; a secondary battery electrically connected with the fuel cell; temperature detection means for detecting a temperature of the fuel cell; first calculation means for calculating a threshold value which represents energy necessary for starting the fuel cell system, based on the temperature of the fuel cell detected by the temperature detection means; wherein the threshold value is used for determining a start-up mode of the fuel cell system; and memory means for storing a value of low-consumption electric power and a value of normal electric power necessary for starting the fuel cell system in a low consumption mode or a normal mode, the first calculation means includes: means for determining a necessary time for the fuel cell to attain a target temperature, based on the temperature of the fuel cell detected by the temperature detection means; and means for calculating a first threshold value representing low-consumption energy and a second threshold value representing normal-consumption energy necessary for starting the fuel cell system in the low consumption mode or in the normal mode, based on the necessary time and the low-consumption electric power or the normal electric power stored in the memory means; wherein the first threshold value is included in the threshold value and used for determining on whether or not to start the fuel cell system; and wherein the second threshold value is included in the threshold value and determining on whether or not the fuel cell system is started in the normal mode or in the low consumption mode is based on data concerning electric energy stored in the secondary battery and the second threshold value, wherein, in the low consumption mode, power generation is started with special limitations placed on the power consumed by system components, wherein the limitations on the power consumed by the system components is implemented by reducing the necessary time to attain the target temperature and by limiting operation of the system components. a fuel cell; a secondary battery electrically connected with the fuel cell; temperature detection means for detecting a temperature of the fuel cell; first calculation means for calculating a threshold value which represents energy necessary for starting the fuel cell system, based on the temperature of the fuel cell detected by the temperature detection means; wherein the threshold value is used for determining a start-up mode of the fuel cell system; and memory means for storing a value of low-consumption electric power and a value of normal electric power necessary for starting the fuel cell system in a low consumption mode or a normal mode, the first calculation means includes: means for determining a necessary time for the fuel cell to attain a target temperature, based on the temperature of the fuel cell detected by the temperature detection means; and means for calculating a first threshold value representing low-consumption energy and a second threshold value representing normal-consumption energy necessary for starting the fuel cell system in the low consumption mode or in the normal mode, based on the necessary time and the low-consumption electric power or the normal electric power stored in the memory means; wherein the first threshold value is included in the threshold value and used for determining on whether or not to start the fuel cell system; and wherein the second threshold value is included in the threshold value and determining on whether or not the fuel cell system is started in the normal mode or in the low consumption mode is based on data concerning electric energy stored in the secondary battery and the second threshold value, wherein, in the low consumption mode, power generation is started with special limitations placed on the power consumed by system components, wherein the limitations on the power consumed by the system components is implemented by reducing the necessary time to attain the target temperature and by limiting operation of the system components. means for determining a necessary time for the fuel cell to attain a target temperature, based on the temperature of the fuel cell detected by the temperature detection means; and means for calculating a first threshold value representing low-consumption energy and a second threshold value representing normal-consumption energy necessary for starting the fuel cell system in the low consumption mode or in the normal mode, based on the necessary time and the low-consumption electric power or the normal electric power stored in the memory means; wherein the first threshold value is included in the threshold value and used for determining on whether or not to start the fuel cell system; and wherein the second threshold value is included in the threshold value and determining on whether or not the fuel cell system is started in the normal mode or in the low consumption mode is based on data concerning electric energy stored in the secondary battery and the second threshold value, wherein, in the low consumption mode, power generation is started with special limitations placed on the power consumed by system components, wherein the limitations on the power consumed by the system components is implemented by reducing the necessary time to attain the target temperature and by limiting operation of the system components.