The corresponding .alpha.-aminooxy ketone is manufactured with a high yield and a high enantioselectivity. A manufacturing method for an optically active .alpha.-aminooxy ketone derivative expressed by formula (1), wherein a ketone expressed by formula (2) is caused to react with a nitroso compound expressed by formula (3) in the presence of a proline derivative expressed by formula (4). ##STR00001## In the formula, R.sup.1 and R.sup.2 respectively denote an alkyl, alkenyl or alkynyl group, and R.sup.1 and R.sup.2 may be linked to form a ring. R.sup.3 denotes an aryl, heterocyclic, alkyl, alkenyl or alkynyl group. A denotes a hydrogen atom, alkoxy group, aryloxy group, acyloxy group or silyloxy group which may have a substituent.