The present invention provides useful compounds for HIV protease inhibitor. A compound represented by formula or its pharmaceutically acceptable salt Formula:wherein ring A isR4 is -Y-Z, hydrogen atom, halogen, hydroxy and the like,R5 is hydrogen atom, halogen, hydroxy and the like,R6 is each independently halogen, hydroxy, carboxy and the like,ring A may be substituted with said R6 at any substitutable position(s),a is an integer of 0 to 7,ring B is substituted or unsubstituted aromatic carbocyclyl, or substituted or unsubstituted aromatic heterocyclyl,ring C is substituted or unsubstituted aromatic carbocyclyl, substituted or unsubstituted non-aromatic carbocyclyl, substituted or unsubstituted aromatic heterocyclyl, or substituted or unsubstituted non-aromatic heterocyclyl,R1 is -Y-Z, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl and the like,R2 and R3 are each independently -Y-Z or hydrogen atom,provided that at least one of R1, R2, R3 and R4 is a group represented by formula: -Y-Z,Y is a bond, or a spacer of any combination selected from the group consisting of -O-, -S-, -NR7-, -C(=O)-, -SO-, -SO2-, -NR7-C(=O)-, -C(=O)-NR7-, -NR7-C(=O)-NR7-, -NR7-C(=O)-O-, -SO2-NR7-, -NR7-SO2-, substituted or unsubstituted alkylene, substituted or unsubstituted alkenylene, substituted or unsubstituted alkynylene, substituted or unsubstituted aromatic carbocyclediyl, substituted or unsubstituted non-aromatic carbocyclediyl, substituted or unsubstituted aromatic heterocyclediyl and substituted or unsubstituted non-aromatic heterocyclediyl,R7 are each independently hydrogen atom, hydroxy, carboxy and the like, andZ is substituted aromatic carbocyclyl, substituted non-aromatic carbocyclyl, substituted aromatic heterocyclyl or substituted non-aromatic heterocyclyl.
