PURPOSE: To change the pressure of a hydraulic fluid in a cylinder so as to miniaturize a brake actuator by providing a complementary gear formation meshed with the gear structure of a piston, and driving the piston along the cylinder by an electric motor.

CONSTITUTION: A brake actuator has a cylinder body, and the cylinder body accommodates therein a piston 2 slidable and sealed to a cylinder internal wall by seals 3, 4. A cylinder includes a space 5 in front of the piston 2. The forward motion of the piston 2 pressurizes a hydraulic fluid in the space 5 and supplies it to an automobile brake through a fluid outlet 6 to actuate the brake. The outer surface of the piston 2 is provided with gear structure in the form of a rack 11 which is meshed with a drive input pinion 12 driven to rotate by an electric motor 13, and the piston 2 is moved forward by the unidirectional rotation of the pinion 12 to pressurize the brake fluid in the chamber 5.