A fuel cell system (100) is provided with a voltage detection device (41) that detects a cell voltage of a cell group containing one or more cells (11), a current density detection device (42) that detects a generated current density of the cell group, and a determination portion (52) that determines the presence or absence of an inflection point of a change in the cell voltage relative to the generated current density based on the detection results of the voltage detection device and the current density detection device.