A fuel cell system comprises a reformer (8) which produces reformate gas
containing hydrogen using gas containing oxygen and raw material vapor,
and a fuel cell (9) which generates power by reacting hydrogen in the
reformate gas with oxygen. A controller (21) determines whether or not
excess hydrogen has been produced by the reformer (8), and when it is
determined that excess hydrogen was produced, burns the excess hydrogen
to keep the reformer (8) warm.