A film forming method for an antireflection film that has a first indium oxide-based thin film and a second indium oxide-based thin film that is laminated on the first indium oxide-based thin film, including a first film forming step that forms the first indium oxide-based thin film by performing sputtering using a first indium oxide-based target in a first reactive gas that contains one, two, or three types selected from a group consisting of oxygen gas, hydrogen gas, and water vapor; and a second film forming step that forms on the first indium oxide-based thin film the second indium oxide-based thin film by performing sputtering using a second indium oxide-based target in a second reactive gas that contains one, two, or three types selected from a group consisting of oxygen gas, hydrogen gas, and water vapor, and that has a different composition from the first reactive gas.