Abstract: A method of assembling a filter element includes the initial step of forming a plastic filter casing with openings from a flat blank with opposite ends thereof bent towards one another and joining these opposite ends together by a sealing seam. Also, a resilient mat filter is folded into a cylindrical shape and pushed open on a fluid-permeable supporting pipe to surround and engage the surrounding pipe. The cylindrical shape of the mat filter has an exterior diameter prior to introduction into the casing that is larger than an interior diameter of the filter casing along the entire axial length of the mat filter. The mat folder is folded into a conical shape to facilitate its introduction thereof into the filter casing. After the axial end is formed, the mat filter and supporting pipe are introduced into the filter casing to define a filter chamber. Finally, two end caps are joined to the filter casing.