Abstract: The filtering apparatus comprises at least one filtering material pleated in fanfold formation and arranged between the inflow or upstream side and the outflow or downstream side. Each pleat open toward the inflow or upstream side possesses a rounded base. The radius of the rounded base is in the range of 0.05 to 0.3 times the pleat depth. Each pleat open toward the outflow or downstream side terminates in an acute angle. A dimensionally stable insert is positioned within each pleat which is open toward the outflow or downstream side. The insert is preferably formed of flat material and comprises bilaterally protuberant deformations. The inserts are provided with bores at least at the protuberant deformations. Each of the inserts space the two confronting downstream sides of the associated filtering pleat and prevent that the filtering material, at high-pressure conditions, is pressed together and becomes impervious at the locations pressed against the inserts.