PURPOSE: To provide a new compd. useful for the treatment and prevention of infectious diseases caused by HIV and for the treatment and prevention of AIDS caused by the diseases.

CONSTITUTION: This inhibitor is a compd. (e.g. 1,6-diphenyl-2(S),5(S)-di(N-(2(S)- N-((quinolin-2-ylmethoxy)carbonyl)aminobutanoyl)amino)-3,4-cis-epoxyhexane) expressed by formula I and its salts. In the formula I, R is alkyl, cycloalkyl, heterocyclic, unsatd. heterocyclic, heterocyclic alkyl, aryl, A-(CH2)qR0 (A is O, NH, S; q is 0 to 3; R0 is cycloalkyl, aryl, heterocyclic or the like) or the like; X is formula II. In the formula II, R1 is R0; R2 is H or alkyl; R3 is aminoacid side chain, CH2CH3, CH2CH2CH3, CH2(CH2)2CH3, C(CH3)3, cyanoalkyl, arylalkyl, (CH2)s-A0-(CH2)r-R4 (A0 is A; R4 is alkyl, cycloalkyl, aryl, heterocyclic or the like; s is 1 to 4; r is 0 to 3) or the like.