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.