A phosphorus-substituted isoprenoid derivative represented by general formula (I) or a pharmacologically acceptable salt thereof, useful for preventing and treating diseases for which a squalene synthetase inhibition is efficacious. In formula (I), R?1 and R2¿ represent each hydrogen, lower alkyl, cycloalkyl, alkenyl, alkynyl, aryl which may be substituted, arylalkyl which may be substituted on the aryl moiety, heteroaryl or heteroarylalkyl; R?3 and R4¿ represent each hydrogen, lower alkyl or alkali metal; Y represents -P(O)(OR5)(OR6) (wherein R?5 and R6¿ represent each hydrogen, lower alkyl or alkali metal) or -CO¿2R?7 (wherein R7 represents hydrogen, lower alkyl or alkali metal); Z represents -(CH¿2?)m- (wherein m represents an integer of 0 or 1 to 3), -(CH2)p-CH=CH-(CH2)q- (wherein m represents 0 or 1 and q represents 1 or 2), or -A-NR?8-(CH¿2)r- (wherein R8 represents hydrogen or lower alkyl, A represents C¿1? to C5 alkylene, and r represents an integer of 0 or 1 to 5); and n represents an integer of 0 or 1 to 5.