A tandem, swash plate-type hydraulic pump in which a drive force transmission system for extending one or more hydraulic pumps and a structure relating to the system can be made simple and light, and one or more hydraulic pumps even when extended can be compactly arranged without increasing the axial length of the pump unit. A drive force take-out mechanism for taking outward part of a driving force out of a spline sleeve member of a spline coupling which is an intermediate coupling of a tandem, swash plate-type hydraulic pump. A first gear is connected with a spline sleeve member and a gear box. The gear box is composed of an intermediate gear, a driven gear, a spline sleeve member, and a gear case for rotatably housing these components. The gear box is removably fixed to an intermediate block and provided on both surfaces of the gear case are pump mounting portions for mounting hydraulic pumps which may be added as appropriate. Suction and discharge channels are disposed vertically in the mounting direction of the drive force take-out mechanism.