A method of manufacturing a thin film transistor (TFT) substrate in which a TFT including an oxide semiconductor layer is formed, the method including: forming an insulating layer to cover the oxide semiconductor layer; and forming an opening in the insulating layer, wherein the insulating layer includes a first film, a second film which is provided above the first film and is an aluminum oxide film, and a third film which is provided above the second film and is a film including silicon, and the forming of an opening includes: forming a resist pattern above the third film; processing the third film by dry etching; and processing the second film by wet etching.