PURPOSE: To provide the subject new compound having nitrogen monoxide synthetase inhibiting action and useful for the treatment of cerebrovascular diseases, especially the symptom of the acute stage of occlusive cerebrovascular disease and the hypotension in sepsis.

CONSTITUTION: This amino acid derivative is a compound expressed by formula I [R1 is H, a (substituted) 1-6C alkoxycarbonyl or a (substituted) 1-9C acyl; R2 is H or a (substituted 1-6C alkyl; R3 is a (substituted) 1-6C alkyl or a (substituted) 2-6C alkeny; R4 is H, R3 and R4 are each a (substituted) 5-7 membered ring; (n) is an integer of 2-4; excluding the case that R3 is methyl and (n) is 3] or its pharmaceutically permissible salt, e.g. δ-(S-(2-fluoroethyl) isothioureido)-L-ylvaline. The compound of formula I can be produced e.g. by reacting a compound of formula II [R1 is a (substituted) 1-6C alkoxycarbonyl, etc.; R2 is a (substituted) 1-6C alkyl] with a compound of formula III (R3 is not methyl when (n) in the formula II is 3; X is a halogen) in a solvent such as acetone at a temperature between room temperature and the boiling point of the reaction mixture.