Abstract: The invention relates to a jar operated by fluid pressure, specially designed for use in drilling or maintenance operations carried out in oil or gas wells. The jar comprises a first (1) and a second (2) end piece, where the second end piece (2) comprises a jar shoulder (15), an inner, cylindrical string part (8) connected to a valve head (9) with a first valve surface (14), and to a weight (12), an outer, cylindrical string part (17), arranged axially movably to the inner cylindrical string part (8), and comprising a second valve surface (30), which is arranged to go into abutment against the first valve surface (14) on the valve head (9). The jarring function is achieved by the inner string part (8) and thereby the weight (12) being set in axial motion and striking the shoulder (15).