The present invention relates to an isoquinoline compound represented by the following formula (I), an optically active form thereof, a pharmaceutically acceptable salt thereof, a water adduct thereof, a hydrate thereof and a solvate thereof, as well as an agent for the prophylaxis and/or treatment of a disease caused by hyperreactivity of poly(ADP-ribose)polymerase, containing the compound, and a poly(ADP-ribose)polymerase inhibitor containing the compound. In addition, this compound is useful as an agent for the prophylaxis and/or treatment of cerebral infarction, particularly as an agent for the prophylaxis and/or treatment of acute cerebral infarction. Furthermore, this compound is useful as a prophylactic and/or therapeutic agent that improves neurological symptoms associated with cerebral infarction, particularly acute cerebral infarction. wherein the symbols are the same as defined in the description.