To simplify the shape and structure of a tank-integrated piston pump 1, increase the capacity, make a pump-case thin and reduce the weight, and prevent the working fluid in the tank from becoming cloudy.

Pump assemblies such as an input rotation-shaft 2, a cylinder block 3, a piston 5, a swash plate 8, and a valve plate 9, etc. are held by a front flange 11, a front case 12, and a rear flange 13. A rear case 14 is provided on a side-face 13b of the front case of the rear flange. A communication hole 15 is made in the rear flange, and a tank is formed. A suction port 16, and connection ports 17a, 17b are provided in the rear flange, and a vent 18 is provided on the outside top-face. An opening 20 is provided on the upper part of a cylinder 19 provided for the cylinder block, the piston, and the swash plate. A return port 17b is allowed to communicate with the communication hole. As for the pump, a swash-plate type unidirectional rotation piston-pump, and a swash-plate type bi-directional rotation piston-pump are applicable. In the case of bi-directional one, a vacuum check-valve is provided on the suction side.