In a fuel cell system that includes a reformer adapted to reform a feedstock, and a fuel cell that uses fuel gas contained in the reformed gas produced by this reformer to generate electricity, aims to improve generation efficiency in the fuel cell through a relatively simple feature. The fuel cell system includes a feedstock supplying section such as a pressurizing pump for supplying the feedstock to the reformer; a burner adapted to combust the fuel gas that was not consumed by electricity generation in the fuel cell, and heat the reformer; a temperature sensor for sensing the temperature of the burner; and a control unit adapted to control on the basis of the sensed temperature the feed rate of the feedstock supplied from the feedstock supplying section to the reformer, so as to maintain the temperature of the reformer within a prescribed temperature range optimized for reforming the feedstock.