A voltage monitor configured for monitoring the voltage of a battery based on a monitor threshold value and configured for carrying out a diagnosis of the monitor threshold value based on first and second diagnostic threshold values, the voltage monitor being characterized by comprising: a first monitor circuit configured for obtaining a first relative correlation between the voltage of the battery and any one of the monitor threshold value and the first diagnostic threshold value, and for outputting first information indicative of the obtained first relative correlation; a second monitor circuit configured for obtaining a second relative correlation between the voltage of the battery and any one of the monitor threshold value and the second diagnostic threshold value, and for outputting second information indicative of the obtained second relative correlation; a threshold switching section configured for: switching a first threshold used by the first monitor circuit to the monitor threshold value and a second threshold used by the second monitor circuit to the monitor threshold value when the voltage monitor carries out the monitor, and switching the first threshold used by the first monitor circuit to the first diagnostic threshold value and the second threshold used by the second monitor circuit to the second diagnostic threshold value when the voltage monitor carries out the diagnosis, the second diagnostic threshold value being the same value as the first diagnostic threshold value; and a fault detecting section configured for: receiving the first information outputted from the first monitor circuit and the second information outputted from the second monitor circuit after the first and second diagnostic threshold values have been switched to the same value; and detecting that there is a fault in one of the first threshold and the second threshold switched by the threshold switching section if the first information of the first monitor circuit is different from the second information of the second monitor circuit. a first monitor circuit configured for obtaining a first relative correlation between the voltage of the battery and any one of the monitor threshold value and the first diagnostic threshold value, and for outputting first information indicative of the obtained first relative correlation; a second monitor circuit configured for obtaining a second relative correlation between the voltage of the battery and any one of the monitor threshold value and the second diagnostic threshold value, and for outputting second information indicative of the obtained second relative correlation; a threshold switching section configured for: switching a first threshold used by the first monitor circuit to the monitor threshold value and a second threshold used by the second monitor circuit to the monitor threshold value when the voltage monitor carries out the monitor, and switching the first threshold used by the first monitor circuit to the first diagnostic threshold value and the second threshold used by the second monitor circuit to the second diagnostic threshold value when the voltage monitor carries out the diagnosis, the second diagnostic threshold value being the same value as the first diagnostic threshold value; and switching a first threshold used by the first monitor circuit to the monitor threshold value and a second threshold used by the second monitor circuit to the monitor threshold value when the voltage monitor carries out the monitor, and switching the first threshold used by the first monitor circuit to the first diagnostic threshold value and the second threshold used by the second monitor circuit to the second diagnostic threshold value when the voltage monitor carries out the diagnosis, the second diagnostic threshold value being the same value as the first diagnostic threshold value; and a fault detecting section configured for: receiving the first information outputted from the first monitor circuit and the second information outputted from the second monitor circuit after the first and second diagnostic threshold values have been switched to the same value; and detecting that there is a fault in one of the first threshold and the second threshold switched by the threshold switching section if the first information of the first monitor circuit is different from the second information of the second monitor circuit.