PROBLEM TO BE SOLVED: To obtain the subject agent effective also against penicillin-resistant strain by formulating aβ-lactam-based antibiotic or salt thereof with aβ- lactamase inhibitor in specific proportions. SOLUTION: This agent is obtained by formulating (A) aβ-lactam-based antibiotic (pref. amoxicillin as a penicillin-based antibiotic) with (B) aβ- lactamase inhibitor (pref. clavulanic acid) in a weight ratio of (1:3) to (13:1), pref. (1:1.5) to (7:1). The fish to be administered with this agent includes Sparidae such as red sea bream and Carangidae such as yellowtail and horse mackerel. This agent is pref. orally administered to cultured fishes and, in this case, it is pref. administered at 20-40 mg/kg for 4 to 7 days in terms of an amoxicillin- based antibiotic.
