In a power supply device including a fuel cell unit with a stack of fuel cells or a fuel cell stack and a secondary battery, an intermittent drive mode is selected under a preset condition to receive power supply only from the secondary battery. The control procedure of the invention determines whether a temporary stop of the fuel cell stack or continuance of the stop of the fuel cell stack leads to potential performance deterioration of the fuel cell stack at a restart of the fuel cell unit. In response to estimation of the potential performance deterioration, the control procedure controls the power supply device to continue or start power generation of the fuel cell stack, regardless of selection of the intermittent drive mode under the preset condition.