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%, Mn of 0.03 mass% to 0.15 mass%, S of 0.01 mass% to 0.05 mass%, optionally Sn or Sb of 0.03 mass% to 0.15 mass%, with the remainder comprising Fe and unavoidable impurities after heating the slab at a temperature not lower than 1320°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 (PH 2 O/PH 2) of 0.15 or below as to not form Fe-based oxides, and providing a mirror-like surface by finish-annealing the steel sheet applied by an annealing separator consisting mainly of alumina, the method for manufacturing grain-oriented silicon steel sheet with mirror-like surface being characterized by controlling the amount of moisture in the annealing separator to not more than 1.5% after application and drying by controlling the water temperatue, stirring time in preparation of the aqueous slurry, BET value and particle size of alumina, wherein the amount of moisture is determined as the loss of mass by annealing to 1000°C after the application and drying, injecting 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 during finish-annealing or injecting inert gas not containing hydrogen and having a dew point of not higher than 0°C as the atmosphere gas 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%, Mn of 0.03 mass% to 0.15 mass%, S of 0.01 mass% to 0.05 mass%, optionally Sn or Sb of 0.03 mass% to 0.15 mass%, with the remainder comprising Fe and unavoidable impurities after heating the slab at a temperature not lower than 1320°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 (PH 2 O/PH 2) of 0.15 or below as to not form Fe-based oxides, and providing a mirror-like surface by finish-annealing the steel sheet applied by an annealing separator consisting mainly of alumina, controlling the amount of moisture in the annealing separator to not more than 1.5% after application and drying by controlling the water temperatue, stirring time in preparation of the aqueous slurry, BET value and particle size of alumina, wherein the amount of moisture is determined as the loss of mass by annealing to 1000°C after the application and drying, injecting 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 during finish-annealing or injecting inert gas not containing hydrogen and having a dew point of not higher than 0°C as the atmosphere gas during finish-annealing.