Disclosed is a device and method for monitoring a common mode voltage. There is provided the device for monitoring a common mode voltage including an input end connected to a node between an energy storage device and a power conversion device, a voltage divider that divides a voltage applied to the input end, a comparator that compares an output of the voltage divider and a reference voltage, and a controller that detects an abnormality in a voltage applied from the power conversion device to the energy storage device based on an output of the comparator.