1. An apparatus for diagnosing a fuel cell, comprising: an impedance measurement unit configured to measure impedance of a fuel cell within a predetermined frequency range; an equivalent circuit model unit configured to derive a parameter value by estimating a predetermined fuel-cell equivalent circuit model in response to the impedance received from the impedance measurement unit; and a fuel-cell-state diagnosis unit configured to diagnose a state of the fuel cell by detecting variation of the parameter value derived from the equivalent circuit model unit, wherein the equivalent circuit model unit derives at least one of parameters of a first ohmic resistor, a second active resistor, a capacitor, and a third resistor, and the equivalent circuit model unit derives parameter values of the second active resistor and the capacitor using a second equivalent circuit model within a second frequency range.