A method of producing a galvannealed steel sheet whereby favorable coating appearance can be obtained with high coating adhesion even in the case of galvannealing a steel strip whose Si content is 0.2 mass% or more, and a decrease in tensile strength can be prevented by lowering the alloying temperature. Mixed gas of humidified gas and dry gas, and dry gas are supplied to a soaking zone in an annealing furnace. The mixed gas is timely supplied from a position of lower half of the soaking zone. The dry gas is timely supplied from near an upper hearth roll in the soaking zone, and furnace gas is timely discharged from a gas discharge port located higher than the upper hearth roll, to control a dew point in at least an uppermost part of the soaking zone to -20 °C or more and 0 °C or less.