Abstract: The hydraulic pump has a rotor which rotates together with an input shaft linked with an automobile's engine, and generates a hydraulic pressure to be applied to a hydraulic actuating apparatus on the automobile. The hydraulic pump also has a transmission unit provided between a rotating shaft of the rotor and the input shaft so as to change a transmission ratio therebetween, and a hydraulic clutch which changes the transmission unit by means of the hydraulic pressure generated at a discharge side of a pump main body so as to apply the pressure to the apparatus. Therefore, the transmission according to an operating state of the apparatus and a rotational speed of the engine can be realized with a simple constitution and no requirement for a sensor for detecting a running state and a controller for controlling according to the detection.