To provide an unsaturated carboxylic acid polymer which is excellent in dispersibility and biodegradability, has high capability of trapping calcium ions, and is useful as a builder for a detergent, its manufacturing method, and a builder, a detergent composition and a dispersant using it.

The unsaturated carboxylic acid polymer has structural units comprising residues of acrylic acid or unsaturated carboxylic acids such as acrylic acid and maleic acid, and structural units in the form in which o-aminophenol is bonded to the methylene main chain, and has a number-average molecular weight of 500 to 20,000,000. Its manufacturing method, and a biodegradable builder, a detergent composition and a dispersant using the polymer are provided.