PROBLEM TO BE SOLVED: To obtain the subject composition having effective and strong antimicrobial power even over O-157:H7 having strong toxicity, useful also for a food by making the composition include an extract of Nemacystus decipiens with hot water, etc. SOLUTION: This antimicrobial composition against Escherichia coli, preferably against pathogenic Escherichia coli O-157 contains an extract of Nemacystus decipiens with hot water and/or its treated substance. The extract of Nemacystus decipiens with hot water is a liquid obtained by extracting a mixture of at least one of especially acystus decipieus, Cladosiphon okamuranus and Tinocladia crassa and water, for example, at <；=60 deg.C for about 30 minutes to 3 hours, heating, filtering the extracted solution and desalting an obtained filtrate by electrodialysis. The treated substance is at least one of a concentrate, a pasted substance, a dried substance, an emulsified substance and a diluted substance. Preferably the obtained composition is a medicine type or a food and beverage type composition.
