An aqueous coating liquid for forming a magnesium oxide film comprising (a) a
magnesium carboxylate derived from a carboxylic acid having 1 to 6 carbon atoms,
(b) a surfactant, (c) a polyhydric alcohol compound, and (d) a solvent made substantially
of water, wherein the magnesium carboxylate is contained in a ratio of 5 wt % or
more and 50 wt % or less. The surfactant is contained in a ratio of 0.01 wt % or
more and 2.5 wt % or less, and the polyhydric alcohol compound is contained in
a ratio of 0.05 wt % or more and 40 wt % or less, based on a weight of the entire
coating liquid. The magnesium carboxylate content is the same with or larger than
that of the polyhydric alcohol compound, and a total weight of the magnesium carboxylate
and the polyhydric alcohol compound is 80 wt % or less of the weight of the entire
coating liquid; and wherein the magnesium carboxylate, the surfactant, and the
polyhydric alcohol compound are dissolved in the solvent.
