A quinolone derivative represented by formula (I): wherein R1 represents a cycloalkyl group; R2 represents a hydrogen atom, an amino group, a hydroxyl group, a thiol group, a halogenomethyl group, an alkyl group, an alkenyl group, an alkynyl group, or an alkoxyl group; R3 represents an amino group, a halogenomethyl group, a halogenomethoxyl group, an alkyl group, an alkenyl group, an alkynyl group, or an alkoxyl group; R4 represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms; R5 represents a cycloalkyl group having 3 to 6 carbon atoms; the above R1 to R5 may be substituted; X represents a halogen atom or a hydrogen atom; and Y represents a hydrogen atom, a phenyl group, an acetoxymethyl group, a pivaloyloxymethyl group, an ethoxycarbonyl group, a choline group, a dimethylaminoethyl group, a 5-indanyl group, a phthalidinyl group, a 5-alkyl-2-oxo-1,3-dioxol-4-ylmethyl group, a 3-acetoxy-2-oxobutyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxymethyl group having 2 to 7 carbon atoms, or a phenylalkyl group having 1 to 6 carbon atoms in the alkyl moiety thereof, or a salt thereof. The compound exhibits high and broad antimicrobial activity against various bacteria including bacteria resistant to drugs and high safety.