PROBLEM TO BE SOLVED: To provide a molten Mg-Zn-based alloy plated steel capable of reconciling adhesiveness with corrosion resistance, while containing high-concentration Mg, concerning a molten metal-based alloy plated steel.SOLUTION: A molten Mg-Zn-based alloy plated steel has an alloy plating layer containing Zn of 15 atom% or more and less than 45 atom%, and having the residue constituted of Mg and inevitable impurities.