An apparatus for treating a medical liquid includes a treatment machine having a coupling surface and a flexible mat arranged on the coupling surface, with a cassette made of a hard part having liquid-conducting passages therein which are covered by a flexible film that is couplable to the coupling surface of the treatment machine via the flexible mat. The apparatus includes an element for monitoring a fault-free coupling to the cassette at the coupling surface by determining a difference of the pressure between the cassette and the flexible mat and the pressure between the flexible mat and the coupling surface. A method for checking the leak tightness of the fault-free coupling employs the apparatus.