An agent for treating heart failure, which improves left ventricular diastolic function itself without depending on the diuretic effect or vasodilation effect； controls the pathological condition of diastolic functional failure； and prevents the recurrence, and can prevent dyspnea and death from the pathological condition, is provided. By directly acting on a heart, 4-[(2-{(2R)-2-[(1E,3S)-4-(4-fluorophenyl)-3-hydroxy-1-buten-1-yl]-5-oxo-1-pyrrolidinyl}ethyl)thio]butanoic acid improves left ventricular diastolic function, and can effectively treat diastolic functional failure among heart failure types. Accordingly, by present invention, a new agent for treating heart failure that can relieve diastolic functional failure, for which no effective therapeutic method has been established, can be provided.
