The water jet of a conventional water jet device is straight, bundled and impinges with its complete cross-section onto the biological structure to be separated. This renders the separating section frequently imprecise and requires an increased flow pressure. Therefore, a water jet device with a novel separating nozzle (14) is furnished. This separating nozzle (14) is furnished with a nozzle channel (15), wherein the nozzle channel (15) is furnished with one or several twisted grooves (16) at the circumference of the nozzle channel (15) and wherein the number of the twisted grooves (16) and the diameter and the length of the nozzle channel (15) are placed in such ratio that the separating jet subjected to pressure is rotated.