A brake device wherein, when a malfunction occurs that renders a slave cylinder (42) inoperable, wheel cylinders (26, 27, 30, 31) are actuated by brake fluid pressure generated by a master cylinder (11), which is actuated by a brake pedal (12). When the rotational speed of a motor (44) of the slave cylinder (42) is greater than a standard value set according to the stroke of the slave cylinder (42), an idling detection means detects idling of the motor (44), making it possible to detect idling of the motor (44) reliably and quickly. When idling of the motor (44) is detected, a control means opens master cut valves (32, 33) and the wheel cylinders (26, 27, 30, 31) are actuated by brake fluid pressure generated by the master cylinder (11), thus enabling the inoperable slave cylinder (42) to be quickly backed up by the master cylinder (11).