The gear pump has a pair of intermeshing gears. The gear (driving wheel) is fixed on the driving shaft. One end of the shaft of the gear pump extends out of the casing and is driven by the prime mover. The other gear (driven wheel) of the gear pump is mounted on the other shaft. When the gear of the gear pump rotates, the liquid enters the suction space along the oil suction pipe, is squeezed by the two gears along the upper and lower shell walls to meet the discharge space (before the teeth mesh with the teeth), and then enters the pressure oil pipe for discharge.