A device for detecting metal with sparking electrosurgical instruments. The device contains a metal detector, which decides, on the basis of the current and voltage delivered to the instrument, whether a spark originating from the instrument contacts biological tissue or a metal part. The component of the current is determined that is inconsistent with a linear equivalent circuit. The elements of the linear equivalent circuit may be determined in a regression calculation. As first decision criterion, the spark characteristic variable Frel is determined from the current. As second decision criterion, a resistance characteristic variable R is determined, which characterizes the tissue resistance. Both characteristic variables are compared with threshold values. If the tissue resistance falls below the resistance threshold value and the spark characteristic variable exceeds the spark size threshold value, a signal is generated that characterizes the activation of the spark against a metal part.