An outbreak of food-poisoning caused by Campylobacter jejuni occured in Omachi city, Nagano prefecture during November and December, 1981. The symptoms common to the 462 patients were abdominal pain (87.2%), diarrhoea (53.9%), fever (42.6%), nausea (28.6%), headache (9.5%) and vomiting (5.6%). Positive rate of isolation of Campylobacter jejuni from the stools of patients in acute stage and convalescent stage was 36.7%, Sera from 6 patients showed elevated agglutinin liters, ranging from 1 : 40 to 1 : 160, against the formalinized antigens of the isolates. Contamination of a lunch served in the school in November 25th was presumed to be the cause of the outbreak. This was the first report on the outbreak of food-poisoning by this microorganism in Nagano prefecture.