An ostomy appliance (1, 101, 201, 501) comprising - a pouch including a front wall (3, 403) and a rear wall - a waste inlet opening (4, 204) in the rear wall for letting the output from the stoma enter into the pouch, - at least one vent opening for letting gas exit the bag, - a filter construction (10, 110, 210, 310, 410, 510, 610) comprising a first and second layer of foil (11, 12, 111, 112, 211, 212, 411, 412, 611, 612) defining an enclosure ∘ the filter construction (10, 110, 210, 310, 410, 510, 610) being attached inside the pouch so that each of the foil-layers (11, 12, 111, 112, 211, 212, 411, 412, 611, 612) are substantially parallel to the front and rear wall (2, 3, 403) of the pouch, the first layer of foil (11, 111, 211, 411, 611) facing the rear wall and the second layer of foil (12, 112, 212, 412,612) facing the front wall (3, 403) of the pouch, ∘ the filter construction (10, 110, 210, 310, 410, 510, 610) being provided with gas-inlets (14; 114, 614) at least in the second foil-layer (12, 112, 212, 412, 612) and a gas-outlet (22, 422) in the second foil placed in alignment with the vent opening - the enclosure including a pre-filter element (15, 115, 215, 415, 615) - the filter construction including a deodorising filter (17, 117, 417, 617) within the enclosure - characterised in that the area of the pre-filter is at least 20 % of the area of the bag when taken in the plane of the bag. - a pouch including a front wall (3, 403) and a rear wall - a waste inlet opening (4, 204) in the rear wall for letting the output from the stoma enter into the pouch, - at least one vent opening for letting gas exit the bag, - a filter construction (10, 110, 210, 310, 410, 510, 610) comprising a first and second layer of foil (11, 12, 111, 112, 211, 212, 411, 412, 611, 612) defining an enclosure ∘ the filter construction (10, 110, 210, 310, 410, 510, 610) being attached inside the pouch so that each of the foil-layers (11, 12, 111, 112, 211, 212, 411, 412, 611, 612) are substantially parallel to the front and rear wall (2, 3, 403) of the pouch, the first layer of foil (11, 111, 211, 411, 611) facing the rear wall and the second layer of foil (12, 112, 212, 412,612) facing the front wall (3, 403) of the pouch, ∘ the filter construction (10, 110, 210, 310, 410, 510, 610) being provided with gas-inlets (14; 114, 614) at least in the second foil-layer (12, 112, 212, 412, 612) and a gas-outlet (22, 422) in the second foil placed in alignment with the vent opening ∘ the filter construction (10, 110, 210, 310, 410, 510, 610) being attached inside the pouch so that each of the foil-layers (11, 12, 111, 112, 211, 212, 411, 412, 611, 612) are substantially parallel to the front and rear wall (2, 3, 403) of the pouch, the first layer of foil (11, 111, 211, 411, 611) facing the rear wall and the second layer of foil (12, 112, 212, 412,612) facing the front wall (3, 403) of the pouch, ∘ the filter construction (10, 110, 210, 310, 410, 510, 610) being provided with gas-inlets (14; 114, 614) at least in the second foil-layer (12, 112, 212, 412, 612) and a gas-outlet (22, 422) in the second foil placed in alignment with the vent opening - the enclosure including a pre-filter element (15, 115, 215, 415, 615) - the filter construction including a deodorising filter (17, 117, 417, 617) within the enclosure - characterised in that the area of the pre-filter is at least 20 % of the area of the bag when taken in the plane of the bag.