<p>Connected midway a primary passage (18), which connects a master cylinder (14) and a braking cylinder (10) to each other, is a discharge side of a pump (16) through an auxiliary passage (20), and provided on a portion of the primary passage (18) between a connection of the primary passage (18) and the auxiliary passage (20) and the master cylinder (14) is a pressure regulating valve (22) adapted to permit a working liquid from the pump (16) to leak to the master cylinder (14) when a discharge pressure of the pump (16) tends to be higher than a set value relative to a master cylinder liquid pressure. The pump (16) is actuated during a braking operation and when it is necessary to generate a higher liquid pressure in the braking cylinder (10) than the master cylinder liquid pressure. <IMAGE></p>
