There is disclosed a tool for centering a casing or liner In a borehole. The tool Includes a tubular section Inserted In the casing/liner. A number of pressure cylinders (4) are mounted in the tubular section along the circumference. The pressure cylinders may be expanded by pressure from the Inside of the tubular section. In use, the casing/liner with the tubular section is run into the borehole with the pressure cylinders retracted. Thereafter, the pressure Inside the casing/liner Is Increased pressing the pressure cylinders out and centering the casing/liner.