A filter unit with a flat particle collecting filter for determining mass concentrations in the air or for emission measurements on air carried in an exhaust gas line, wherein the filter unit can be traversed by air in order to collect the particles contained in the air on the particle collecting filter, the filter unit comprising at least: - a module housing for the particle collecting filter, which is formed from a multipart housing assembly (1, 3) with a first housing part and a second housing part, wherein the two housing parts (1, 3) are connected to each other, and - a support sieve for supporting the particle collecting filter within the housing assembly (1, 3), which is arranged on the second housing part, characterized in that the filter unit furthermore comprises a filter insert which is accommodated in the housing assembly (1, 2) and which includes a carrier component on which the particle collecting filter is held, wherein the carrier component includes a first frictional locking area and a second frictional locking area, wherein the two frictional locking areas (210, 211) are different and each protrude substantially perpendicularly from a filter plane defined by the flat particle collecting filter, wherein the filter insert is frictional attached to the first housing part via the first frictional locking area and to the second housing part via the second frictional locking area so that when the two housing parts (1, 3) are separated from each other, the filter insert with the particle collecting filter still is held on one of the housing parts (1, 3), wherein the support sieve is pressed against the underside of the particle collecting filter when the two housing parts (1, 3) and the filter insert are connected to each other, and wherein the particle collecting filter is loosely inserted into a cutout of the carrier component and the filter unit is configured in such a way that the particle collecting filter is fixed on the filter insert and within the housing assembly (1, 3) defined by the two housing parts (1, 3) only when the two housing parts (1, 3) are connected to each other as intended. - a module housing for the particle collecting filter, which is formed from a multipart housing assembly (1, 3) with a first housing part and a second housing part, wherein the two housing parts (1, 3) are connected to each other, and - a support sieve for supporting the particle collecting filter within the housing assembly (1, 3), which is arranged on the second housing part, characterized in that the filter unit furthermore comprises a filter insert which is accommodated in the housing assembly (1, 2) and which includes a carrier component on which the particle collecting filter is held, wherein the carrier component includes a first frictional locking area and a second frictional locking area, wherein the two frictional locking areas (210, 211) are different and each protrude substantially perpendicularly from a filter plane defined by the flat particle collecting filter, wherein the filter insert is frictional attached to the first housing part via the first frictional locking area and to the second housing part via the second frictional locking area so that when the two housing parts (1, 3) are separated from each other, the filter insert with the particle collecting filter still is held on one of the housing parts (1, 3), wherein the support sieve is pressed against the underside of the particle collecting filter when the two housing parts (1, 3) and the filter insert are connected to each other, and wherein the particle collecting filter is loosely inserted into a cutout of the carrier component and the filter unit is configured in such a way that the particle collecting filter is fixed on the filter insert and within the housing assembly (1, 3) defined by the two housing parts (1, 3) only when the two housing parts (1, 3) are connected to each other as intended. characterized in that the filter unit furthermore comprises a filter insert which is accommodated in the housing assembly (1, 2) and which includes a carrier component on which the particle collecting filter is held, wherein the carrier component includes a first frictional locking area and a second frictional locking area, wherein the two frictional locking areas (210, 211) are different and each protrude substantially perpendicularly from a filter plane defined by the flat particle collecting filter, wherein the filter insert is frictional attached to the first housing part via the first frictional locking area and to the second housing part via the second frictional locking area so that when the two housing parts (1, 3) are separated from each other, the filter insert with the particle collecting filter still is held on one of the housing parts (1, 3), wherein the support sieve is pressed against the underside of the particle collecting filter when the two housing parts (1, 3) and the filter insert are connected to each other, and wherein the particle collecting filter is loosely inserted into a cutout of the carrier component and the filter unit is configured in such a way that the particle collecting filter is fixed on the filter insert and within the housing assembly (1, 3) defined by the two housing parts (1, 3) only when the two housing parts (1, 3) are connected to each other as intended.