A fuel cell electric power generating system is provided which does not occupy a large space, which requires a lower initial cost for equipment than the prior art, and of which the running cost is low. The system includes a reformer producing hydrogen-rich gas by utilizing a source gas, source gas supplying means of supplying the source gas to the reformer, air supplying means of supplying purging air to the reformer, and a fuel cell generating electric power by utilizing the hydrogen-rich gas produced at the reformer and air for electric power generation supplied from outside, wherein in stopping the operation of the fuel cell, the supply of the source gas to the reformer is stopped and the hydrogen-rich gas remaining within the fuel cell electric power generating system, steam and the purging air are passed in this order.