A method of treating steel for a grain-oriented electrical steel sheet, comprising: performing slab reheating of a slab for a grain-oriented electrical steel sheet obtained by continuous casting; performing hot-rolling of the slab to obtain a hot-rolled steel strip; performing annealing of the hot-rolled steel strip to obtain an annealed steel strip in which a primary inhibitor has precipitated; cold-rolling the annealed steel strip once or more to obtain a cold-rolled steel strip; performing decarburization annealing of the cold-rolled steel strip to obtain a decarburization-annealed steel strip in which primary recrystallization has been caused; nitriding the decarburization-annealed steel strip in a mixed gas of hydrogen, nitrogen and ammonia while running the decarburization-annealed steel strip to obtain a nitrided steel strip in which a secondary inhibitor has been introduced; applying an annealing separating powder containing MgO as a main component to the nitrided steel strip; and performing finish annealing of the nitrided steel strip to cause secondary recrystallization, wherein a surface temperature of the slab is decreased down to 600 °C or lower between start of the continuous casting and start of the slab reheating, wherein the surface temperature of the slab is held at 150 °C or higher between the start of the continuous casting and the start of the slab reheating, and wherein the surface temperature of the slab in the slab reheating is set to not lower than 1080 °C and not higher than 1200 °C, wherein the steel which is cast by continuous casting comprises a composition consisting of C: 0.025 mass% to 0.09 mass%, Si: 2.5 mass% to 4.0 mass%, Mn: 0.05 mass% to 0.15 mass%, acid-soluble Al: 0.022 mass% to 0.033 mass%, and N: 0.005 mass% to 0.010 mass%, S equivalent of 0.004 mass% to 0.015 mass%, and the balance composed of Fe and inevitable impurities, wherein the S equivalent is a value found by Expression " + 0.405[Se]" where the S content is and the Se content is [Se], and optionally, one or more selected from Sb, Sn and P: 0.02 mass% to 0.30 mass%, Cu: 0.05 mass% to 0.30 mass%, Cr: 0.02 mass% to 0.3 mass%, Ni: 0.02 mass% to 0.3 mass%, a total of Mo and Cd: 0.008 mass% to 0.3 mass%, and a content of Ti is not more than 0.005 mass%. performing slab reheating of a slab for a grain-oriented electrical steel sheet obtained by continuous casting; performing hot-rolling of the slab to obtain a hot-rolled steel strip; performing annealing of the hot-rolled steel strip to obtain an annealed steel strip in which a primary inhibitor has precipitated; cold-rolling the annealed steel strip once or more to obtain a cold-rolled steel strip; performing decarburization annealing of the cold-rolled steel strip to obtain a decarburization-annealed steel strip in which primary recrystallization has been caused; nitriding the decarburization-annealed steel strip in a mixed gas of hydrogen, nitrogen and ammonia while running the decarburization-annealed steel strip to obtain a nitrided steel strip in which a secondary inhibitor has been introduced; applying an annealing separating powder containing MgO as a main component to the nitrided steel strip; and performing finish annealing of the nitrided steel strip to cause secondary recrystallization, wherein a surface temperature of the slab is decreased down to 600 °C or lower between start of the continuous casting and start of the slab reheating, wherein the surface temperature of the slab is held at 150 °C or higher between the start of the continuous casting and the start of the slab reheating, and wherein the surface temperature of the slab in the slab reheating is set to not lower than 1080 °C and not higher than 1200 °C, wherein the steel which is cast by continuous casting comprises a composition consisting of C: 0.025 mass% to 0.09 mass%, Si: 2.5 mass% to 4.0 mass%, Mn: 0.05 mass% to 0.15 mass%, acid-soluble Al: 0.022 mass% to 0.033 mass%, and N: 0.005 mass% to 0.010 mass%, S equivalent of 0.004 mass% to 0.015 mass%, and the balance composed of Fe and inevitable impurities, wherein the S equivalent is a value found by Expression " + 0.405[Se]" where the S content is and the Se content is [Se], and optionally, one or more selected from Sb, Sn and P: 0.02 mass% to 0.30 mass%, Cu: 0.05 mass% to 0.30 mass%, Cr: 0.02 mass% to 0.3 mass%, Ni: 0.02 mass% to 0.3 mass%, a total of Mo and Cd: 0.008 mass% to 0.3 mass%, and a content of Ti is not more than 0.005 mass%.