A fuel reforming system, comprising a reformer (2, 3, 4) which produces reformate gas from rich raw fuel gas, and supplies the reformate gas to the fuel cell (28) during a reforming operation, a burner (1) which produces lean combustion gas, and supplies the lean combustion gas to the reformer (2, 3, 4) during a warmup operation of the reformer (2, 3, 4), and a nonflammable fluid supply apparatus which supplies a nonflammable fluid other than fuel and air to the reformer (2, 3, 4). During the warmup operation of the reformer (2, 3, 4), the lean combustion gas is supplied from the burner (1) to the reformer (2, 3, 4), and when warmup of the reformer (2, 3, 4) is complete, the nonflammable fluid is supplied from the nonflammable fluid supply apparatus to the reformer (2, 3, 4), and the reforming operation of the reformer (2, 3, 4) then starts.