PURPOSE: To remarkably improve the yield of a product by applying an Fe-P alloy plating having a specified P content to a steel sheet before galvannealing.

CONSTITUTION: The face with low Zn coating weight in a steel sheet is applied with Fe-P alloy plating contg. 0.1 to 15wt.% P and is thereafter applied with differential thickness galvannealing. By applying the Fe-P alloy plating as the same manner, a galvannealing reaction at the time of galvannealing by using an Al-added Zn bath can be suppressed, and a uniform δ1 layer can be obtd. In the case the content of P in the Fe-P alloy plating exceeds the upper limit, the adhesion of the Fe-P alloy plating itself is deteriorated, and after the galvannealing treatment, the δ1 phase uniformly develops, and an insufficient result is shown. In the case the content of P in the Fe-P alloy plating is less than the lower limit, a Γ1 phase develops on the boundary of the base-Zn plating at the time of the galvannealing treatment, by which the content of the δ1 phase is reduced.