Prophylactic or therapeutic agent for diseases attributable to infection with helicobacters

The present invention relates to an agent for the prophylaxis and treatment of diseases caused by Helicobacter infections, which comprises (S)-1-cyclopropyl-1,4-dihydro-7-Ä2-(N,N-dimethylaminomethyl)morpholino Ü-6-fluoro-8-methoxy-4-oxoguinoline-3-carboxylic acid or a pharmaceutically acceptable salt thereof. The prophylactic and therapeutic agent of the present invention is effective even when used alone in a small dose for a short time, is almost free of problematic side effects such as tolerance and diarrhea, and is low toxic and capable of safe and ensured bacterial eradication. It is useful for the prophylaxis and treatment of diseases caused by Helicobacter infections, particularly, gastritis, gastric ulcer, duodenal ulcer, malignant lymphoma and gastric cancer.