The invention relates to an apparatus for treating a medical liquid which 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 which are covered by a flexible film being able to be coupled to the coupling surface of the treatment machine via the flexible mat. In accordance with the invention, the apparatus for making possible the monitoring of a fault-free coupling to the cassette at the coupling surface is made such that a pressure difference of the pressure can be determined between the cassette and the flexible mat and of the pressure between the mat and coupling surface. The invention furthermore relates to a method for the checking of the leak tightness of the aforesaid apparatus.