PURPOSE: To provide a HIV protease inhibitor useful for prophylaxis of infection by HIV, treatment for infective diseases by HIV and/or treatment for AIDS.

CONSTITUTION: This compd. is expressed by formula I, wherein R is a cycloalkyl, heterocyclic group, aryl, unsatd. heterocyclic group; X is a single bond, (CH2)q, O-(CH2)q, (CH2)q-O, NR5-(CH2)m (q is 1 to 3; m is 0 to 3); n is 0 to 2; R1 is an aryl, cycloalkyl; R2 is an amino acid side chain, unsatd. heterocyclic group, alkylaminocarbonyl-alkenediyl, CH2-R, etc.; Y is an aryl, unsatd. heterocyclic group; R3 is CO-NR4R4, formulae II to IV (p is 4, 5; R4 is H, an alkyl, etc.; R5, R6 are R4, an alkoxy, amino, etc.), etc. The compd. is, for example, [1S-(1e*,4e*,5S*)]-N-(1-(2-amino-2-oxoethyl)-2-oxo-3-aza-4-phenylmethyl-5- hydroxy-6-(2-(1-t-butylamino-1-oxomethyl)phenyl)hexyl)-2-quinonylcarboxamide.