An apparatus for controlling the hydraulic circuit of a piston diaphragm pump, having at least a pump diaphragm alternatingly subjected to the presure or the suction stroke of a piston operatingly connected to driving means, the apparatus comprising control means changing the amount of hydraulic medium displaced by the piston in dependence of a power demand or the like. A displacement sensor is associated with one side of the diaphragm facing the hydraulic medium, the sensor measuring the stroke travel of the diaphragm and generating a corresponding stroke travel signal which is transmitted to control means, the control means including storage means storing predetermined stroke values and/or stroke value ranges for predetermined hydraulic power values of the pump. The control means compare the stroke travel signal with the predetermined stroke values or stroke value ranges and generate a control signal if the measured stroke travel deviates from the predetermined value or value range to change the amount of hydraulic medium per time unit and/or to generate an alarm signal in dependence of the control signal.