PURPOSE: To provide a brake actuator which is small in its size, has high response and has no fear of running out of oil film in a seal.

CONSTITUTION: A chamber R2 is sectioned by a piston 8 which has a large diameter seal 8a and a small diameter seal 8b, in a control cylinder 7 which consists of a large diameter cylinder 7a and a small diameter cylinder 7b. A screw shaft 9a is built in the control cylinder and a nut 9b engaged with the shaft is displaced by its rotation. The piston 8 to which the nut 9b is fixed moves thereby, volume of the chamber R2 varies and oil pressure to a wheel cylinder 19 is increased and decreased while the chamber R2 is cut off from other chamber or a master cylinder by valves 4, 5. No space is required because no piston rod is required weight reduction is achieved response is improved and the rod does not go in and out. Displacement distance of the seal section is large and oil film does not run out, because the screw shaft is of a rotation displacement type.