Provided is a therapeutic agent for heart failure which can improve the diastolic function of the left ventricle without relying on the diuretic activity or the vasodilatory activity thereof and therefore can control the condition of impaired vasodilatation so as to prevent the recurrence of impaired vasodilatation and can also improve breathing difficulty or death associated with the condition of impaired vasodilatation. 4 [(2 {(2R) 2 [(1E 3S) 4 (4 Fluorophenyl) 3 hydroxy 1 butene 1 yl] 5 oxo 1 pyrrolidinyl}ethyl)thio]butanoic acid can act on the heart directly to improve the diastolic function of the left ventricle and therefore can treat heart failure particularly impaired vasodilatation effectively. Thus the present invention can provide a novel therapeutic agent for heart failure which can improve impaired vasodilatation for which any effective treatment method is not established yet.
