The invention relates to a gastroscope comprising an insertion tube (2) in which a working channel (3) is arranged, and a sensor (4) which is guided in the working channel (3) via a guide wire (5). The sensor (4) has a first electrode (6) which is produced of an acid-fast noble metal, and a second electrode (7) which is produced of silver. An electrical voltage can be applied between the first electrode (6) and the second electrode (7) and a change in an electric variable can be measured between the first electrode (6) and the second electrode (7) when ammonia is present. The gastroscope according to the invention allows the screening of the gastric acid and of the tissue of the stomach lining for Helicobacter pylori in a manner that is gentle on the patient.
