A fuel cell system capable of performing activation for stabilizing electrical characteristics of a fuel cell with suppressing generation of polarity inversion associated with drying of a polymer electrolyte membrane and excess power consumption. An activation method for a fuel cell, the fuel cell system including a fuel cell having a fuel electrode and an oxidizer electrode that are provided on both sides of a polymer electrolyte membrane; a resistance detector for detecting an internal resistance of the fuel cell; a load connection portion having a mechanism for connecting a resistor between the fuel electrode and the oxidizer electrode; and a control unit for controlling the load connection portion. The control unit controls the operation of the load connection portion based on a value of the inner resistance of the fuel cell, which is detected by the resistance detector.