Provided is a Zn-Al-Mg-Si-based plated steel sheet which has excellent
chemical conversion treatability, suppressed swelling under a
coating film, and
corrosion resistance to a
dissimilar metal joint. A Zn-Al-Mg-Si plated steel sheet according to the present invention has a base steel sheet and a plating layer formed on at least one surface of the base steel sheet, and is characterized in that the plating layer has a Zn-Al-MgZn2
ternary eutectic structure and a composition containing, in
mass%, 0.50-3.00% of Mg, 0.10-3.00% of Al, 0.010-0.20% of Si, and 0.30% or less of Fe, with the remainder being Zn and unavoidable impurities, and in that the plating layer has a Zn-Al-Mg-Si
ternary eutectic structure. In a
backscattered electron image having a
magnification of 5000 times obtained using a
scanning electron microscope at an
acceleration voltage of 7.0 kV, in a
ternary eutectic structure, needle-like Mg2Si
alloy phases are present in a
number density of less than 100,000 / mm2, and needle-like Mg2Si
alloy phases are present in a
number density of at least 500 / mm2 when the same operation is performed at an
acceleration voltage of 15.0 kV, and the
number density of the needle-like Mg2Si
alloy phases is less than 100,000 / mm2 when the same operation is performed at an
acceleration voltage of 15.0 kV, and the number density of the needle-like Mg2Si alloy phases is less than 100,000 / mm2. The number density of the needle-like SiO2
oxide phases is more than 100000 / mm < 2 >.