An ECU (19) outputs control signals to an electric booster (16) of a fluid-pressure generating device (5) and to an ESC (22). In this way, the establishment and cutting off of communication through pressure-increase control valves (29), (29′), (31), and (31′) of the ESC (22) are controlled to connect each of wheel cylinders (1) to (4) in turn to the master cylinder (8). In doing so, the electric booster (16) is controlled for each of the wheel cylinders (1) to (4) to measure a stroke of a piston (10), (11) of a master cylinder (8), or a volume of brake fluid proportional to the stroke of the piston (10), (11), and a fluid pressure detected by fluid-pressure sensors (20) and (21). The fluid volume-pressure characteristic of each of the wheel cylinders (1) to (4) so determined is used for comparison to detect any atmospheric air in each pipeline.