Disclosed is an oxide semiconductor having an amorphous structure, wherein higher mobility and reduced carrier concentration are achieved. Also disclosed are a thin film transistor, a method for producing the oxide semiconductor, and a method for producing the thin film transistor. Specifically disclosed is an oxide semiconductor which is characterized by being composed of an amorphous oxide represented by the following a general formula: Inx+1MZny+1SnzO(4+1.5x+y+2z) (wherein M is Ga or Al, 0≦x≦1, −0.2≦y≦1.2, z≧0.4 and 0.5≦(x+y)/z≦3). This oxide semiconductor is preferably subjected to a heat treatment in an oxidizing gas atmosphere after film formation. Also specifically disclosed is a thin film transistor which is characterized by comprising the oxide semiconductor.