In a cell voltage detecting system detecting cell voltages of a fuel cell connected in series, a voltage detector detects cell voltages. An auxiliary power supply generates a voltage equal to or greater than a minimum voltage for driving the voltage detector at positive and negative terminals thereof. First and second diodes are connected to a positive drive terminal of the voltage detector to supply the power to the drive terminal of the voltage detector from a high voltage side between the fuel cell or the auxiliary power supply. A negative terminal of the fuel cell, the negative drive terminal, and the negative terminal of the auxiliary power supply are connected to have a same potential. A method of maintaining a drive voltage for the voltage detector is also disclosed in which the drive voltage for the voltage detector is supplied from the high voltage side.