A filter comprising - a cylindrical body and an inlet channel for liquid to be filtered leading into the body and an outlet channel for filtered liquid leading out of the body, wherein the body includes a removable, body-closing cap - a central tube, through which the filtered liquid is discharged, - in the body, a coaxially cylindrical main filtering element supplied with the liquid to be filtered from the inlet channel, the liquid to be filtered being received on the outside of the main filtering element and a cylindrical reserve filtering element (19.1; 19.2), that is shorter than the main filtering element, is positioned inside the main filtering element and whose filtering resistance is not more than equal to that of the main filtering element, - a by-pass valve including spring-operated closure means and a closure means spring, wherein - the spring-operated closure means has - a closed position blocking the flow of the liquid to be filtered from bypassing the main filtering element and allowing the flow of the liquid to be filtered to the main filtering element and further through the main filtering element and - an open position allowing the liquid to be filtered to bypass the main filtering element to the reserve filtering element (19.1; 19.2) and further to flow through the reserve filtering element (19.1; 19.2), and wherein - the closure means spring urges the closure means to the closed position and which gives way for opening the closure means when the pressure difference across the main filtering element exceeds a given value, characterized in that - the body-closing cap, when in attachment with the body, presses the closure means against the closure means spring by way of the main filtering element or the reserve filtering element (19 1, 19 2), - the main filtering element and the reserve filtering element (19.1; 19.2) are integrated to form single filter element (16.1; 16.2) and interconnected at one end thereof, - the filter element (16.1; 16.2) comprises an overhead lid at its end, - the filter further comprises a central stem attached to the central tube, wherein the removable cap is attached to the stem with a fastening peg between the stem and the removable cap, - the closure means spring is a coil spring fitted around the stem and - the closure means is a cup-shaped piston having its interior sealed against the central stem or the fastening peg, the cup-shaped piston having a top rim placed against the overhead lid present at the end of the filter element (16.1; 16.2) in the closed position of the closure means. - a cylindrical body and an inlet channel for liquid to be filtered leading into the body and an outlet channel for filtered liquid leading out of the body, wherein the body includes a removable, body-closing cap - a central tube, through which the filtered liquid is discharged, - in the body, a coaxially cylindrical main filtering element supplied with the liquid to be filtered from the inlet channel, the liquid to be filtered being received on the outside of the main filtering element and a cylindrical reserve filtering element (19.1; 19.2), that is shorter than the main filtering element, is positioned inside the main filtering element and whose filtering resistance is not more than equal to that of the main filtering element, - a by-pass valve including spring-operated closure means and a closure means spring, - the spring-operated closure means has - a closed position blocking the flow of the liquid to be filtered from bypassing the main filtering element and allowing the flow of the liquid to be filtered to the main filtering element and further through the main filtering element and - an open position allowing the liquid to be filtered to bypass the main filtering element to the reserve filtering element (19.1; 19.2) and further to flow through the reserve filtering element (19.1; 19.2), and wherein - the closure means spring urges the closure means to the closed position and which gives way for opening the closure means when the pressure difference across the main filtering element exceeds a given value, - the body-closing cap, when in attachment with the body, presses the closure means against the closure means spring by way of the main filtering element or the reserve filtering element (19 1, 19 2), - the main filtering element and the reserve filtering element (19.1; 19.2) are integrated to form single filter element (16.1; 16.2) and interconnected at one end thereof, - the filter element (16.1; 16.2) comprises an overhead lid at its end, - the filter further comprises a central stem attached to the central tube, wherein the removable cap is attached to the stem with a fastening peg between the stem and the removable cap, - the closure means spring is a coil spring fitted around the stem and - the closure means is a cup-shaped piston having its interior sealed against the central stem or the fastening peg, the cup-shaped piston having a top rim placed against the overhead lid present at the end of the filter element (16.1; 16.2) in the closed position of the closure means.