To obtain a medicine for prophylaxis of sudden death such as heart diseases, aortic anueurysm, cerebral apoplexy and Pokkuri diseases (a so-called sudden manhood death syndrome).

This prophylactic agent for the sudden death comprises a compound represented by formula (I) (wherein, R1 denotes hydrogen atom or hydroxy group) or its acid addition salt or a hydrate thereof. The compound represented by formula (I) or its acid addition salt or the hydrate thereof effectively suppresses serotonin-induced coronary arterial hypercontracture for a pathologic model animal of the Pokkuri diseases. Thereby, the prophylactic drug for the sudden death is effective in the prophylaxis of the sudden death resulting from the heart diseases, Pokkuri diseases, and the like, as causative diseases.