A method of manufacturing bags that are rectangular when collapsed and which comprise two superimposed flat sides, sealed at the edges, and a handle member. The method comprises the steps of: laying out a web forming three parallel subordinate webs, one extending longitudinally in the middle and one on each lateral side thereof; folding two top flanks out of the middle subordinate web to create a handle-top portion; V-folding a center strip of the middle subordinate web to create a handle member; providing the handle member with a finger slot and with depressions that allow the middle part to be pulled apart along the handle member to form a gusset; bringing the two lateral subordinate webs together; and attaching the lateral subordinate webs together along seams to create bag edges and cutting off the bags along their edges.