To carry out a mucosa resection, a liquid is injected beneath a tissue which is then separated by means of mechanical or electrosurgical equipment. A device is provided that includes a facility for endoscopic water-jet surgery with a control device for regulating a pressure at which a water jet is discharged from a nozzle of the facility to obtain a predetermined amount of energy. An adjusting assembly is provided for setting the pressure to a first value and to a second value. The first value is so chosen that the water jet penetrates a mucosa in such a way that the mucosa is lifted from an underlying muscularis through formation of a fluid deposit. The second value is adjusted and different from the first value in such a way that the mucosa is sectionally cut through by the water jet.