A control device for a vehicle, comprising: a motor generator mechanically connected to an output shaft of an engine; a first battery electrically connected to the motor generator; a second battery electrically connected to an electric load of the vehicle; a relay (43a, 43b) that is able to electrically connect the first battery to the second battery when the relay (43a, 43b) is turned on or off; and a relay control unit that turns off the relay (43a, 43b) during a power operation of the motor generator so as to disconnect an electric connection between the first battery and the second battery, the relay control unit turning on the relay (43a, 43b) during a regenerative operation of the motor generator so as to electrically connect the first battery to the second battery, characterized in that the control device further comprises: detecting means (51, 72) for detecting either a voltage of the second battery or charge current of the second battery; and a relay failure diagnosing unit that determines that the relay (43a, 43b) is off-fixed, which means that the relay (43a, 43b) continues to be turned off even if the relay control unit is commanding the relay (43a, 43b) to be turned on during the regenerative operation of the motor generator, when the voltage of the second battery detected by the detecting means (51, 72) is a predetermined voltage or lower or the charge current of the second battery detected by the detecting means (51, 72) is a predetermined current or smaller. a motor generator mechanically connected to an output shaft of an engine; a first battery electrically connected to the motor generator; a second battery electrically connected to an electric load of the vehicle; a relay (43a, 43b) that is able to electrically connect the first battery to the second battery when the relay (43a, 43b) is turned on or off; and a relay control unit that turns off the relay (43a, 43b) during a power operation of the motor generator so as to disconnect an electric connection between the first battery and the second battery, the relay control unit turning on the relay (43a, 43b) during a regenerative operation of the motor generator so as to electrically connect the first battery to the second battery, characterized in that the control device further comprises: detecting means (51, 72) for detecting either a voltage of the second battery or charge current of the second battery; and a relay failure diagnosing unit that determines that the relay (43a, 43b) is off-fixed, which means that the relay (43a, 43b) continues to be turned off even if the relay control unit is commanding the relay (43a, 43b) to be turned on during the regenerative operation of the motor generator, when the voltage of the second battery detected by the detecting means (51, 72) is a predetermined voltage or lower or the charge current of the second battery detected by the detecting means (51, 72) is a predetermined current or smaller. detecting means (51, 72) for detecting either a voltage of the second battery or charge current of the second battery; and a relay failure diagnosing unit that determines that the relay (43a, 43b) is off-fixed, which means that the relay (43a, 43b) continues to be turned off even if the relay control unit is commanding the relay (43a, 43b) to be turned on during the regenerative operation of the motor generator, when the voltage of the second battery detected by the detecting means (51, 72) is a predetermined voltage or lower or the charge current of the second battery detected by the detecting means (51, 72) is a predetermined current or smaller.