A diagnostic device diagnosing a state of a fuel cell in which a plurality of cells generating electric energy by using an electrochemical reaction of an oxidizing agent containing oxygen and a fuel containing hydrogen are stacked, the diagnostic device being characterized by comprising: signal superimposition means configured to superimpose an alternating current of a low-frequency on an output current of the fuel cell; first voltage detection means configured to detect each of voltages output from the respective cells; second voltage detection means configured to detect a voltage output from the fuel cell; current detection means configured to obtain a current flowing through the plurality of cells; first calculation means configured to calculate a cell impedance of any of the plurality of cells with respect to the alternating current based on the current detected by the current detection means and the voltage detected by the first voltage detection means; second calculation means (520, S100) configured to calculate a reference impedance based on a stack overall impedance of the entire fuel cell or an impedance of a reference cell, wherein the stack overall impedance of the entire fuel cell is calculated with respect to the alternating current based on the current detected by the current detection means and the voltage detected by the second voltage detection means and the reference cell is selected in advance among the plurality of cells as one of the cells in which a hydrogen deficiency is unlikely to occur; the diagnostic device further comprising first determination means (530, S110) configured to determine whether an absolute value of the cell impedance calculated by the first calculation means exceeds an absolute value of the reference impedance, wherein the first determination means is configured to diagnose that any cell is in a state of the hydrogen deficiency when the first determination means determines that the absolute value of the cell impedance calculated by the first calculation means exceeds the absolute value of the reference impedance. signal superimposition means configured to superimpose an alternating current of a low-frequency on an output current of the fuel cell; first voltage detection means configured to detect each of voltages output from the respective cells; second voltage detection means configured to detect a voltage output from the fuel cell; current detection means configured to obtain a current flowing through the plurality of cells; first calculation means configured to calculate a cell impedance of any of the plurality of cells with respect to the alternating current based on the current detected by the current detection means and the voltage detected by the first voltage detection means; second calculation means (520, S100) configured to calculate a reference impedance based on a stack overall impedance of the entire fuel cell or an impedance of a reference cell, wherein the stack overall impedance of the entire fuel cell is calculated with respect to the alternating current based on the current detected by the current detection means and the voltage detected by the second voltage detection means and the reference cell is selected in advance among the plurality of cells as one of the cells in which a hydrogen deficiency is unlikely to occur; the diagnostic device further comprising first determination means (530, S110) configured to determine whether an absolute value of the cell impedance calculated by the first calculation means exceeds an absolute value of the reference impedance, wherein the first determination means is configured to diagnose that any cell is in a state of the hydrogen deficiency when the first determination means determines that the absolute value of the cell impedance calculated by the first calculation means exceeds the absolute value of the reference impedance.