PURPOSE: To produce a filter insert having longitudinally elongated ribs for maintaining the spacings formed in the folding parts of a filter material by deforming the corrugated filter material heated to a specific temp. by the jaws of a forming machine, then heating the filter material to a specific temp. by heating jaws.

CONSTITUTION: The tape-like filter material 20 transported in a longitudinal direction for manufacturing the filter insert 4 is contracted by corrugation by as much as the required additional material quantity necessary for the longitudinally elongated ribs 15. The filter material 20 heated to the temp. below the shrinkage temp. of the filter material 20 and above the deformation temp. thereof is subjected to permanent deformation free from internal stresses between the jaws 23 to 26 of the forming machine and is then cooled down to the deformation temp. or below, by which the longitudinally elongated ribs 15 are formed. Further, the linear range of the filter material 20 disposed in order to form folding edges 7 to 14 is heated to the temp. between the deformation temp. and melting temp. by the heating jaws 30, 31 until the rugged characteristic induced by the ribs 15 of the filter material 20 and/or the corrugated deformation thereof is compensated. The filter material is then cooled down to the deformation temp. or below.