Disclosed is a cassette for a medical fluid (5), used for treating medical fluid (5) with gas (19) and/or emptied by gas (19) pressure, containing at least one receiving chamber (1) for reception of medical fluid (5) and at least one supplying chamber (3) for supplying gas, in which at least one hydrophobic filtering device with filtering surface for purification and supply of gas (19) into receiving chamber (1) is placed. Supplying chamber (3) is provided with at least one connector (17) for supplying gas and in the process of cassette application is located above receiving chamber (1), which has connection (35) for reception or removal of medical fluid. Vector of normal to filtering surface of hydrophobic filtering device passes non-parallel to vector of normal to mirror (7) of medical fluid (5), located in receiving chamber (1). A device for treating medical fluid, containing the disclosed cassette, is also disclosed. Realisation of the invention prevents destruction of hydrophobic filtering device as a result of hydroshock, which might result from sudden and excessive supply of liquid into the receiving chamber.
