Power supply control device and a power supply control method for a hybrid vehicle

When the temperature of an engine (ENG) and a main battery (B1) is low at the time of activating a vehicle system, an ECU (40) starts the engine (ENG) in advance and outputs a running permission signal after completion of engine startup. At this stage, the ECU (40) suspends a defect diagnosis operation on a subsidiary load (60) and feedback-controls the DC/DC converter (30) by setting the voltage control value to a level lower than the output voltage from the subsidiary battery (B2) and at least the lower limit of the operating voltage of the ECU (40). The ECU (40) feedback-controls the DC/DC converter (30) by setting the voltage control value to a level of at least the output voltage of the subsidiary battery (B2) in response to completion of engine startup. Then, the suspension of the defect diagnosis operation on the subsidiary load (60) is canceled and a running permission signal is output.