The present invention provides a surface protective agent having good hard coating properties and good adhesion properties to metal. The surface protective agent comprises: a polymer comprising a reaction product obtained by reacting (a) a silane compound containing an amino group represented by the following formula: R4-n-Si-(OR')n (wherein R represents an organic group containing an amino group; R' represents methyl, ethyl or propyl; and n is an integer selected from 1 to 3) with (b) at least one boron compound selected from the group consisting of H3BO3 and B2O3, in an amount of the component (b) being 0.02 mol or larger per mol of the component (a); (c) a metal alkoxide and/or a condensation product of a metal alkoxide; and (d) a phosphorus compound.