PURPOSE: To prevent a large fluctuation in the flow amount of the pump by a method wherein three sets of delivery ports are arranged with the intervals of 60° and pistons, whose number of set is the multiple of six, are incorporated into the pump.

CONSTITUTION: A valve plate 5 is formed with a suction port 6 as well as first, second and third delivery ports 7, 8, 9 while a side plate 10 is formed with the suction port 11 as well as the delivery ports 12, 13, 14 respectively. The delivery ports 7, 8, 9 are arranged on the same circumference with the intervals of 60° with respect to the center 5' of the valve plate while the pistons 3, whose number of set is the multiple of six, are incorporated into the pump. According to this method, the fluctuation of flow amount in case the pistons 3 discharge pressure oil to respective delivery ports may be reduced.