A proline ester represented by the following formula (I): wherein R1 represents a hydroxy-lower alkyl group, a lower alkoxy-lower alkyl group, or a lower alkoxy-lower alkoxy-lower alkyl group or a pharmaceutically acceptable salt thereof. The proline ester (I) of the present invention and a pharmaceutically acceptable salt thereof are useful as a prodrug of enalaprilat, which is a medicine useful for preventing or treating circulatory diseases such as hypertension, cardiac diseases (e.g., cardiac hypertrophy, cardiac failure, and myocardial infarct), nephritis, and apoplexy. Thus, a drug containing the ester or a salt thereof is preferably formulated to a percutaneous preparation, particularly a patch, from the viewpoint of medicinal activity and use.