Abstract: An air filter with a pleated filter medium and a mechanism to keep the pleats in equal spacing and defined position. The air filter comprises a pleated filter medium, adhesively delimited to both sides in pleating direction by two U-shaped pleat-lock strips, which hold the pleats in place. The filter medium including the pleat-lock strips is covered with a die-cut frame that wraps around the two sides of the filter medium perpendicular to pleating direction. Alternatively, the pleat-lock strips may be planar and adhesively attached to the outside edges at suitable distances to the edges of the top side of the filter medium in pleating direction. The filter medium including the pleat-lock strips is case-like enclosed by two die-cut frame pieces, one on each side of the filter medium. One frame piece wraps the edges of the filter medium in pleating direction; the other one wraps the edges of the filter medium perpendicular to pleating direction.