Primary alcohols are selectively converted into aldehydes by using a Nb(2)O(5) photocatalyst under visible-light irradiation. A strong interaction between the alcohol and Nb(2)O(5) generates a donor level within the forbidden band of Nb(2)O(5), which provides a visible-light-harvesting ability. Over oxidation of aldehydes into carboxylic acids does not proceed under visible-light irradiation.