Provided is an oxide semiconductor configured to be used in a thin film transistor having high field-effect mobility; a small shift in threshold voltages against light and bias stress; excellent stress resistance. The oxide semiconductor has also excellent resistance to a wet-etchant for patterning of a source-drain electrode. The oxide semiconductor comprises In, Zn, Ga, Sn and O, and satisfies the requirements represented by expressions (1) to (4) shown below, wherein [In], [Zn], [Ga], and [Sn] represent content (in atomic %) of each of the elements relative to the total content of all the metal elements other than oxygen in the oxide. (1.67×[Zn]+1.67×[Ga])≧100 (1) {([Zn]/0.95)+([Sn]/0.40)+([In]/0.4)}≧100 (2) [In]≦̸40 (3) [Sn]≧5 (4)
