A method for manufacturing grain-oriented silicon steel sheet with mirror-like surface having good iron loss properties, comprising the steps of: preparing hot-rolled steel sheet by hot-rolling silicon steel slab comprising Si of 0.8 mass % to 4.8 mass %, C of 0.003 mass % to 0.1 mass %, acid-soluble Al of 0.012 mass % to 0.05 mass %, N of not more than 0.01 mass %, with the remainder substantially comprising Fe and unavoidable impurities after heating the slab at a temperature not higher than 1280°C, reducing the hot-rolled sheet, as rolled or after annealing, to a final sheet thickness by applying one or two or more cold rollings, with intermediate annealing interposed, forming an oxidized layer consisting mainly of silica on the surface of the cold-rolled steel sheet by implementing decarburized-annealing in an atmosphere gas of such degree of oxidation as to not form Fe-based oxides, by controlling the degree of oxidation (PH 2 O/PH 2) of the atmosphere to 0.01 to 0.15 in a temperature range of 800°C to 850°C, applying a nitriding treatment, subsequently applying an annealing separator consisting mainly of alumina and providing a mirror-like surface by finish-annealing the steel sheet, wherein the amount of moisture carried in the annealing separator consisting mainly of alumina after application and drying of an aqueous slurry is controlled to not more than 1.5%, and an atmosphere gas having a degree of oxidation (PH 2 O/PH 2) of not lower than 0.0001 and not higher than 0.2 as the atmosphere gas is injected during finish-annealing or an inert gas having a dew point of not higher than 0°C as the atmosphere gas is injected during finish-annealing. preparing hot-rolled steel sheet by hot-rolling silicon steel slab comprising Si of 0.8 mass % to 4.8 mass %, C of 0.003 mass % to 0.1 mass %, acid-soluble Al of 0.012 mass % to 0.05 mass %, N of not more than 0.01 mass %, with the remainder substantially comprising Fe and unavoidable impurities after heating the slab at a temperature not higher than 1280°C, reducing the hot-rolled sheet, as rolled or after annealing, to a final sheet thickness by applying one or two or more cold rollings, with intermediate annealing interposed, forming an oxidized layer consisting mainly of silica on the surface of the cold-rolled steel sheet by implementing decarburized-annealing in an atmosphere gas of such degree of oxidation as to not form Fe-based oxides, by controlling the degree of oxidation (PH 2 O/PH 2) of the atmosphere to 0.01 to 0.15 in a temperature range of 800°C to 850°C, applying a nitriding treatment, subsequently applying an annealing separator consisting mainly of alumina and providing a mirror-like surface by finish-annealing the steel sheet, wherein the amount of moisture carried in the annealing separator consisting mainly of alumina after application and drying of an aqueous slurry is controlled to not more than 1.5%, and an atmosphere gas having a degree of oxidation (PH 2 O/PH 2) of not lower than 0.0001 and not higher than 0.2 as the atmosphere gas is injected during finish-annealing or an inert gas having a dew point of not higher than 0°C as the atmosphere gas is injected during finish-annealing.