A phenomenon of change of a contact resistance between an oxide semiconductor and a metal depending on an oxygen content ratio in introduced gas upon depositing an oxide semiconductor film made of indium gallium zinc oxide, zinc tin oxide, or others in an oxide semiconductor thin-film transistor. A contact layer is formed with an oxygen content ratio of 10% or higher in a region from a surface, where the metal and the oxide semiconductor are contacted, down to at least 3 nm deep in depth direction, and a region to be a main channel layer is further formed with an oxygen content ratio of 10% or lower, so that a multilayered structure is formed, and both of ohmic characteristics to the electrode metal and reliability such as the suppression of threshold potential shift are achieved.