An apparatus for treating wound with negative pressure. The apparatus including a wound cover (1), a first pump (9) for providing said negative pressure to the wound (5), a canister (6), a first conduit (8) between the wound cover (1) and the canister, means for measuring the pressure within the canister (6), and a second conduit (10) which connects the canister with the first pump. A circulating pump (12) is arranged to transport gas from the canister (6) to the wound region via a third conduit (13) and back to the canister (6) via the first conduit (8) in order to press out a possible liquid column from the first conduit (8) to the canister (6). The invention further relates to a method for controlling the negative pressure in a wound with use of said apparatus.
