An abnormality diagnostic device for diagnosing an abnormality in a MOSFET switch element, the abnormality diagnostic device comprising: a current sensor detecting current that flows through a battery module; a first MOSFET switch element that causes a charge current to flow into the battery module and is connected to the battery module in series; a second MOSFET switch element that causes a discharge current to flow out of the battery module and is connected to the first MOSFET switch element and the battery module in series; a charge relay that is provided in a current path through which a charge current flows from a charger to the battery module; and a control unit that turns off, after the charging of the battery module performed by the charger is completed, the first MOSFET switch element in a state in which the charge relay is on, transmits, to the charger, an instruction signal to start a charging operation, and determines, on the basis of an output signal of the current sensor, whether current is flowing through the first MOSFET switch element due to the charging operation of the charger, so as to diagnose a short circuit abnormality in the first MOSFET switch element on the basis of a result of the determination; that turns off, after the charging is completed, the second MOSFET switch element in a state in which the charge relay is on, and determines whether a discharge current is flowing through the second MOSFET switch element, so as to diagnose a short circuit abnormality in the second MOSFET switch element; and that turns off, after the diagnoses of a short circuit abnormality in the first MOSFET switch element and a short circuit abnormality in the second MOSFET switch element are completed, the charge relay in a state in which the first MOSFET switch element and the second MOSFET switch element are on, and determines whether a discharge current is flowing through the charge relay, so as to diagnose a short circuit abnormality in the charge relay. a current sensor detecting current that flows through a battery module; a first MOSFET switch element that causes a charge current to flow into the battery module and is connected to the battery module in series; a second MOSFET switch element that causes a discharge current to flow out of the battery module and is connected to the first MOSFET switch element and the battery module in series; a charge relay that is provided in a current path through which a charge current flows from a charger to the battery module; and a control unit that turns off, after the charging of the battery module performed by the charger is completed, the first MOSFET switch element in a state in which the charge relay is on, transmits, to the charger, an instruction signal to start a charging operation, and determines, on the basis of an output signal of the current sensor, whether current is flowing through the first MOSFET switch element due to the charging operation of the charger, so as to diagnose a short circuit abnormality in the first MOSFET switch element on the basis of a result of the determination; that turns off, after the charging is completed, the second MOSFET switch element in a state in which the charge relay is on, and determines whether a discharge current is flowing through the second MOSFET switch element, so as to diagnose a short circuit abnormality in the second MOSFET switch element; and that turns off, after the diagnoses of a short circuit abnormality in the first MOSFET switch element and a short circuit abnormality in the second MOSFET switch element are completed, the charge relay in a state in which the first MOSFET switch element and the second MOSFET switch element are on, and determines whether a discharge current is flowing through the charge relay, so as to diagnose a short circuit abnormality in the charge relay. that turns off, after the charging of the battery module performed by the charger is completed, the first MOSFET switch element in a state in which the charge relay is on, transmits, to the charger, an instruction signal to start a charging operation, and determines, on the basis of an output signal of the current sensor, whether current is flowing through the first MOSFET switch element due to the charging operation of the charger, so as to diagnose a short circuit abnormality in the first MOSFET switch element on the basis of a result of the determination; that turns off, after the charging is completed, the second MOSFET switch element in a state in which the charge relay is on, and determines whether a discharge current is flowing through the second MOSFET switch element, so as to diagnose a short circuit abnormality in the second MOSFET switch element; and that turns off, after the diagnoses of a short circuit abnormality in the first MOSFET switch element and a short circuit abnormality in the second MOSFET switch element are completed, the charge relay in a state in which the first MOSFET switch element and the second MOSFET switch element are on, and determines whether a discharge current is flowing through the charge relay, so as to diagnose a short circuit abnormality in the charge relay.