A solid oxide fuel cell comprising: a fuel cell module furnished with multiple fuel cell units; a fuel supply device for supplying fuel to the fuel cell module; an oxidant gas supply device for supplying oxidant gas to the fuel cell module; and a controller for controlling the fuel cell module operation by controlling the amount of fuel supplied from the fuel supply device; characterized in that the controller is furnished with a degradation determining circuit for determining degradation in the fuel cell module from predetermined measurement value obtained from the results of fuel cell module operation, and with a degradation response circuit for changing fuel cell module operating conditions to respond to degradation when it is determined using the degradation determining circuit that there has been a degradation of a fuel cell module; and wherein, when the degradation determining circuit determines there has been a fuel cell module degradation, the controller is configured to operate the fuel cell module under operating conditions changed by the degradation response circuit and thereafter is configured to obtain and store specified measurement values obtained from the operating results of the fuel cell module under the changed operating conditions, and by comparing the stored measurement values with subsequently newly acquired specified measurement values, is configured to determine the additional amount of progress in the degradation of the fuel cell module occurring after the operating conditions changed, while on the other hand is configured to not update the stored measurement value if it is determined that there is no degradation of the fuel cell module. a fuel cell module furnished with multiple fuel cell units; a fuel supply device for supplying fuel to the fuel cell module; an oxidant gas supply device for supplying oxidant gas to the fuel cell module; and a controller for controlling the fuel cell module operation by controlling the amount of fuel supplied from the fuel supply device; characterized in that the controller is furnished with a degradation determining circuit for determining degradation in the fuel cell module from predetermined measurement value obtained from the results of fuel cell module operation, and with a degradation response circuit for changing fuel cell module operating conditions to respond to degradation when it is determined using the degradation determining circuit that there has been a degradation of a fuel cell module; and wherein, when the degradation determining circuit determines there has been a fuel cell module degradation, the controller is configured to operate the fuel cell module under operating conditions changed by the degradation response circuit and thereafter is configured to obtain and store specified measurement values obtained from the operating results of the fuel cell module under the changed operating conditions, and by comparing the stored measurement values with subsequently newly acquired specified measurement values, is configured to determine the additional amount of progress in the degradation of the fuel cell module occurring after the operating conditions changed, while on the other hand is configured to not update the stored measurement value if it is determined that there is no degradation of the fuel cell module.