Abstract: A ring-shaped corrugated filter element blank having radially alternate peak and valley folds is placed on a first mold having as many first concentric projections as the number of the peak folds of the ring-shaped blank and as many first concentric grooves as the number of the valley folds of the ring-shaped blank, with the peak folds fitted respectively over the first projections and the valley folds fitted respectively in the first grooves. Then, the ring-shaped blank is pressed between the first mold and a second mold, the second mold having second projections lying along concentric lines fitting respectively in the first grooves with the valley folds received therein, and second concentric grooves receiving respectively the first projections with the peak folds fitted thereover. The ring-shaped blank is heated while it is being pressed between the first and second molds, thereby forming a ring-shaped filter element. Thereafter, the ring-shaped filter element is cooled.