An automatic brake device is provided with: a battery (16) connected to a power supply line (14); a fluid pressure unit (6) driven by receiving electric power supply via the power supply line to provide a brake with a brake fluid pressure; an alternator (10) that generates a voltage higher than a stored voltage of the battery by receiving power from an engine of a vehicle; a capacitor (11) that stores electric power generated by the alternator; and a DC/DC converter (13) that converts a stored voltage of the capacitor to a desired value supplied to the power supply line. When the vehicle is in danger of contacting a forward obstacle, an output voltage of the DC/DC converter is set to a target voltage higher than normal before the brake is activated, or, if the output voltage of the DC/DC converter cannot be set to the target voltage, the alternator is driven to connect the voltage generated by the alternator to the power supply line.