A housing of a hydraulic pump is configured to have a servo unit detachably attached thereto so as to control tilt direction and angle of its movable swash plate. A port block is formed therein with a pair of main ports and, and a pair of main fluid passages and fluidly connecting respective main ports and to cylinders in its cylinder block. Main ports are used to have respective external pipes connected thereto so as to fluidly connect the hydraulic pump to a hydraulic motor disposed outside of the hydraulic pump, thereby constituting a hydrostatic transmission.