Abstract: A filter and gas vent system incorporated in an ostomy bag during its manufacture includes a filter in the shape of a flat band, its deodorizing core being left open to the ambient air at two ends of the filter. The latter is held inside the bag by two weld seams joining walls of the bag to each other. A key feature of the invention is that each of the two weld seams extends from one of two side edges of the bag to its upper edge. The filter can thus be contained entirely in the bag, which thus isolates the ends from the outside. The material from which the core of the filter is made, often activated charcoal, no longer risks soiling the clothes or being driven off when the subject is showering. This invention also prevents the liquid matter collected in the bag from seeping out by passing along the core of the filter.