To industrially advantageously obtain the subject compound having a high optical purity by reducing an asymmetric ketone by using an asymmetric reducing agent obtained by treating an optically active β-amino-alcohol with a boron hydride in the presence of a specific compound.

An asymmetric ketone (e.g. acetophenone, etc.), is reduced by using an asymmetric reducing agent obtained by treating an optically active β-amino-alcohol of the formula [R1 is H, a lower alkyl or a (substituted) aralkyl; R2 to R5 are each H, a lower alkyl, a (substituted) aryl or a (substituted) aralkyl; * is an asymmetric carbon atom] (e.g. optically active norephedrine, etc.), with a boron hydride (e.g. diborane, etc.), in the presence of a phosphoric ester (e.g. trimethyl phosphate, etc.), a phosphorous ester (e.g. trimethyl phosphite, etc.), or a lower alcohol (e.g. methanol, ethanol, etc).