A positive displacement pump apparatus includes a pump chamber in a pipe system for receiving a pumping fluid, the pipe system having an inlet and an outlet which can be shut by a valve. The chamber is connected via an intermediate fluid chamber with a displacement element including a piston head and a piston rod, which is arranged to alternately carry out a suction stroke and a displacement stroke during its movement to displace fluid in the intermediate fluid chamber, thereby increasing or reducing the volume of the pump chamber. A flexible separating element including a diaphragm is provided at the pump chamber to separate the fluid in the intermediate fluid chamber from the pumping fluid. A force member is provided for applying force to the piston head during the displacement stroke to counteract force exerted on the piston head by the fluid in the intermediate fluid chamber.