A hydraulic pump comprises: a drive shaft that receives power from a power source; a housing journaling the drive shaft; a port block assembled with the housing; a cylinder block attached slidably rotatably to the port block and disposed in the housing; plungers fitted into respective cylinders formed in the cylinder block reciprocally slidably parallel to the drive shaft; a movable swash plate rotatably supported by the housing and abuts against the plungers, the movable swash plate including a trunnion shaft portion projecting outward from the housing; and an arm provided on the trunnion shaft portion outside of the housing, characterized in that: the housing is configured so that a servo unit for adjusting tilt direction and angle of the movable swash plate can be detachably attached to the housing; the arm is configured so that a piston in a servo housing of the servo unit can be connected to the arm when the servo unit is attached to the housing, and so that a mechanical linkage can be connected to the arm when the servo unit is not attached to the housing; the port block includes a pair of externally open ports (Pb, Pe), which are adaptable to be fluidly connected via respective external pipes to a hydraulic motor disposed separately from the hydraulic pump; and the port block includes a pair of main fluid passages (ML1, ML2) connecting the respective externally open ports (Pb, Pe) to the cylinders in the cylinder block. a drive shaft that receives power from a power source; a housing journaling the drive shaft; a port block assembled with the housing; a cylinder block attached slidably rotatably to the port block and disposed in the housing; plungers fitted into respective cylinders formed in the cylinder block reciprocally slidably parallel to the drive shaft; a movable swash plate rotatably supported by the housing and abuts against the plungers, the movable swash plate including a trunnion shaft portion projecting outward from the housing; and an arm provided on the trunnion shaft portion outside of the housing, characterized in that: the housing is configured so that a servo unit for adjusting tilt direction and angle of the movable swash plate can be detachably attached to the housing; the arm is configured so that a piston in a servo housing of the servo unit can be connected to the arm when the servo unit is attached to the housing, and so that a mechanical linkage can be connected to the arm when the servo unit is not attached to the housing; the port block includes a pair of externally open ports (Pb, Pe), which are adaptable to be fluidly connected via respective external pipes to a hydraulic motor disposed separately from the hydraulic pump; and the port block includes a pair of main fluid passages (ML1, ML2) connecting the respective externally open ports (Pb, Pe) to the cylinders in the cylinder block. the housing is configured so that a servo unit for adjusting tilt direction and angle of the movable swash plate can be detachably attached to the housing; the arm is configured so that a piston in a servo housing of the servo unit can be connected to the arm when the servo unit is attached to the housing, and so that a mechanical linkage can be connected to the arm when the servo unit is not attached to the housing; the port block includes a pair of externally open ports (Pb, Pe), which are adaptable to be fluidly connected via respective external pipes to a hydraulic motor disposed separately from the hydraulic pump; and the port block includes a pair of main fluid passages (ML1, ML2) connecting the respective externally open ports (Pb, Pe) to the cylinders in the cylinder block.