A galvannealed steel sheet comprising: a steel sheet; a galvannealed layer which is formed on at least one surface of the steel sheet and includes an amount equal to or more than 0.05 mass% and equal to or less than 0.5 mass% of Al, an amount equal to or more than 6 mass% and equal to or less than 12 mass% of Fe, an amount equal to or less than 2 mass% of at least one of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and rare earth elements as needed, and a balance composed of Zn and inevitable impurities; and a composite oxide layer of Mn, Zn, and P which is formed on a surface of the galvannealed layer, includes an amount equal to or more than 0.1 mg/m 2 and equal to or less than 100 mg/m 2 of Mn, an amount equal to or more than 1 mg/m 2 and equal to or less than 100 mg/m 2 of P, and Zn, has a P/Mn ratio of equal to or higher than 0.3 and equal to or lower than 50, wherein the galvannealed layer consists of a flat portion having an area ratio of equal to or higher than 10% and equal to or lower than 70% and a rough portion which is at a position closer to the steel sheet than the flat portion at an interface of the galvannealed layer and the composite oxide layer, the composite oxide layer of Mn, Zn, and P contains an amorphous compound, a surface roughness Ra of the rough portion is equal to or higher than 0.5 μm and equal to or lower than 10 μm, a surface roughness Ra of the flat portion is equal to or higher than 0.01 μm and lower than 0.5 μm, and the surface roughness Ra of the flat portion and the surface roughness Ra of the rough portion are obtained by measuring the arithmetic average roughness Ra specified in JIS B0601:1994 of a range of 10 μm × 10 μm using a laser microscope at 10 points and averaging the measured values. a steel sheet; a galvannealed layer which is formed on at least one surface of the steel sheet and includes an amount equal to or more than 0.05 mass% and equal to or less than 0.5 mass% of Al, an amount equal to or more than 6 mass% and equal to or less than 12 mass% of Fe, an amount equal to or less than 2 mass% of at least one of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and rare earth elements as needed, and a balance composed of Zn and inevitable impurities; and a composite oxide layer of Mn, Zn, and P which is formed on a surface of the galvannealed layer, includes an amount equal to or more than 0.1 mg/m 2 and equal to or less than 100 mg/m 2 of Mn, an amount equal to or more than 1 mg/m 2 and equal to or less than 100 mg/m 2 of P, and Zn, has a P/Mn ratio of equal to or higher than 0.3 and equal to or lower than 50, wherein the galvannealed layer consists of a flat portion having an area ratio of equal to or higher than 10% and equal to or lower than 70% and a rough portion which is at a position closer to the steel sheet than the flat portion at an interface of the galvannealed layer and the composite oxide layer, the composite oxide layer of Mn, Zn, and P contains an amorphous compound, a surface roughness Ra of the rough portion is equal to or higher than 0.5 μm and equal to or lower than 10 μm, a surface roughness Ra of the flat portion is equal to or higher than 0.01 μm and lower than 0.5 μm, and the surface roughness Ra of the flat portion and the surface roughness Ra of the rough portion are obtained by measuring the arithmetic average roughness Ra specified in JIS B0601:1994 of a range of 10 μm × 10 μm using a laser microscope at 10 points and averaging the measured values.