A system (e.g., a monitoring device) for monitoring a first battery (e.g., a vehicle battery) provided in a vehicle is provided with a monitoring unit for monitoring the electric state of the first battery (e.g., the vehicle battery) capable of starting a motor of the vehicle; and a power supply unit that is a second battery for supplying electric power to the monitoring unit. The monitoring unit may determine whether or not the motor is stopped, and when the motor is stopped, the monitoring unit may monitor, as the electric state of the first battery, e.g., the voltage of the first battery.