PURPOSE: To obtain the subject antithrombotic agent having excellent safety and useful for the treatment of ischemic cerebrovascular disorder, myocardial infarction, etc., by using a specific isoquinoline sulfonamide derivative, etc., as an active component.

CONSTITUTION: The objective antithrombotic agent contains, as an active component, the substituted isoquinoline sulfonamide derivative of formula [R1 is H, Cl or OH; when R1 is H, A is (substituted) 2-6C alkylene, R2 is H, R3 is H, 1-6C alkyl, aryl, etc., and R4 is H, 1-6C alkyl, aryl, benzoyl, etc.; when R1 is Cl or OH, A is (substituted) 2-6C alkylene, R2 and R3 are H, 1-6C alkyl, etc., and R4 is H, 1-6C alkyl or amidino] or its acid addition salt, e.g. 1-(5- isoquinolinesulfonyl)homopiperazine. The daily dose of the agent is preferably 0.01-20mg/kg for parenteral administration and 0.02-40mg/kg for oral administration.