PURPOSE: To obtain the subject new compound excellent in the inhibitory action on a nitrogen monoxide synthase found in intracerebral neurocytes and in the selectivity of the enzyme, and useful as a therapeutic agent for cerebrovascular lesions such as cerebral infarction, cerebral hemorrhage, subarachnoid hemorrhage and cerebral edema.

CONSTITUTION: This compound (its salt) is expressed by formula I, and is obtained by either hydrolysis or deprotection through direct treatment with a deprotecting agent (e.g. trifluoroacetic acid, hydrochloric acid) of a compound of formula II [R1 is a (substituted) lower alkoxycarbonyl or (substituted) lower acyl; R2 is a (substituted) l-6C (branched) alkyl] [e.g. α-N-tbutoxycarbonyl-β-(S- methylisothioureido)-L-norvaline-t-butyl ester]. It is preferable that the deprotection is conducted at room temperature in the absence of water by using trifluoroacetic acid.