The present invention provides an etchant which is usable for a long period of time owing to its high indium solubility and reduced precipitation of a salt of oxalic acid and indium, and further has excellent residue removal and antifoaming properties, thereby being suitable for etching indium oxide-based films. An etchant for etching an indium oxide-based film, the etchant containing: (A) oxalic acid; (B) polyvinylpyrrolidone; and (C) water.