A method for fabricating a field-effect transistor having a gate electrode, a source electrode, a drain electrode, and an active layer forming a channel region, the active layer having an oxide semiconductor mainly containing magnesium and indium, the method comprising: a deposition step of depositing an oxide film; a patterning step of patterning the oxide film by processes including etching to obtain the active layer; and a heat-treatment step of heat-treating the obtained active layer subsequent to the patterning step, wherein in said heat treatment step a heat-treat temperature for heating the obtained active layer is equal to or higher than a deposition temperature at which the oxide film is deposited in the deposition step; and characterized by: depositing, in the deposition step, the oxide film having a composition of: a)In 2 MgO 4, or b) a material in which at least one of aluminium (Al) and gallium (Ga) is substituted for part of indium (In) in the oxide film having a composition of In 2 MgO 4, or c) a material in which, at least one of calcium (Ca), strontium (Sr) and barium (Ba) is substituted for part of magnesium (Mg) in the oxide film having a composition of In 2 MgO 4, or d) a material in which at least one of nitrogen and fluorine is substituted for part of oxygen in the oxide film having a composition of In 2 MgO 4, wherein in the deposition step, said oxide film is deposited as a polycrystalline film such that at least part of said oxide film includes a spinel structure or an olivine structure. a deposition step of depositing an oxide film; a patterning step of patterning the oxide film by processes including etching to obtain the active layer; and a heat-treatment step of heat-treating the obtained active layer subsequent to the patterning step, wherein in said heat treatment step a heat-treat temperature for heating the obtained active layer is equal to or higher than a deposition temperature at which the oxide film is deposited in the deposition step; and characterized by: depositing, in the deposition step, the oxide film having a composition of: a)In 2 MgO 4, or b) a material in which at least one of aluminium (Al) and gallium (Ga) is substituted for part of indium (In) in the oxide film having a composition of In 2 MgO 4, or c) a material in which, at least one of calcium (Ca), strontium (Sr) and barium (Ba) is substituted for part of magnesium (Mg) in the oxide film having a composition of In 2 MgO 4, or d) a material in which at least one of nitrogen and fluorine is substituted for part of oxygen in the oxide film having a composition of In 2 MgO 4, wherein in the deposition step, said oxide film is deposited as a polycrystalline film such that at least part of said oxide film includes a spinel structure or an olivine structure. depositing, in the deposition step, the oxide film having a composition of: a)In 2 MgO 4, or b) a material in which at least one of aluminium (Al) and gallium (Ga) is substituted for part of indium (In) in the oxide film having a composition of In 2 MgO 4, or c) a material in which, at least one of calcium (Ca), strontium (Sr) and barium (Ba) is substituted for part of magnesium (Mg) in the oxide film having a composition of In 2 MgO 4, or d) a material in which at least one of nitrogen and fluorine is substituted for part of oxygen in the oxide film having a composition of In 2 MgO 4, a)In 2 MgO 4, or b) a material in which at least one of aluminium (Al) and gallium (Ga) is substituted for part of indium (In) in the oxide film having a composition of In 2 MgO 4, or c) a material in which, at least one of calcium (Ca), strontium (Sr) and barium (Ba) is substituted for part of magnesium (Mg) in the oxide film having a composition of In 2 MgO 4, or d) a material in which at least one of nitrogen and fluorine is substituted for part of oxygen in the oxide film having a composition of In 2 MgO 4, wherein in the deposition step, said oxide film is deposited as a polycrystalline film such that at least part of said oxide film includes a spinel structure or an olivine structure.