A process of the present invention produces a hydroxylactone by subjecting an unsaturated carboxylic acid having a double bond not conjugated to a carboxyl group or an ester thereof to (i) a reaction with hydrogen peroxide in the presence of a metallic compound containing a metallic element selected from W, Mo, V and Mn or (ii) a reaction with a peroxide containing the metallic element to thereby yield a corresponding hydroxylactone having a hydroxyl group combined with one of carbon atoms constituting the double bond and being cyclized at the other carbon atom position. The metallic compound may be one selected from oxides, oxoacids and salts thereof. The unsaturated carboxylic acid includes, for example, ,-unsaturated carboxylic acids, ,-unsaturated carboxylic acids, and ,-unsaturated carboxylic acids. The process can produce hydroxylactones in high yields at low cost.