A hydraulic modulator includes a cylinder for slidably supporting a power piston and changing the volume of a hydraulic chamber communicating with brake calipers according to the sliding of the power piston, and an electric motor for sliding the power piston through a power transmitting mechanism. The cylinder extends parallel to a rotating shaft, and a rotational motion of the electric motor is converted into a linear motion on an axis parallel to the rotating shaft. The power transmitting mechanism is provided by a ball screw mechanism composed of a ball screw nut, balls, and a ball screw shaft for converting the rotational motion of the electric motor into the linear motion of the push slider. The push slider has a head portion coming into spherical contact with the power piston. The hydraulic modulator so configured has a small size, and an improved arrangement of components.