The invention relates to a filter provided with a bypass opening (14) to be used especially as an oil filter in hydraulic systems. The filter includes a casing-like filtering element (1) mounted on a support (3), whereby the stream to be filtered can be conducted into the space (11) defined by the element such that filtering takes place from this space through the filtering element envelope towards the outside of the space. The bypass opening (14) is produced by prestressing the filtering element (1) and the support against each other with a spring (5) such that, as the pressure in the filter exceeds the prestressing force, they will be dislodged relative to each other, leaving the opening concerned between the support and the filtering element (1). Essential features of the filter of the invention are that the filter is equipped with a frame (17) between the casing-like filtering element (1) and the support (3), that the frame and the support are prestressed against each other to leave a bypass opening (14) between them under the effect of pressure, and that the filtering element is removably attached to the prestressed frame. The frame (17) and the support (3) are kept continuously prestressed in the filter, while the cylindrical filtering element (1) attached to the frame, which does not affect prestressing, can be removed and replaced when needed.